tests: have combine_logs default to most recent test dir

Summary:
Have combine_logs.py default to the most recent test directory if no argument is provided.
Backport of Core [[bitcoin/bitcoin#14683 | PR14683]]

Test Plan:
```
ninja
../test/functional/feature_bip68_sequence.py --configfile=./test/config.ini
../test/functional/combine_logs.py -c
```

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7795

test/functional/combine_logs.py

@@ -2,7 +2,9 @@
 """Combine logs from multiple bitcoin nodes as well as the test_framework log.
 
 This streams the combined log output to stdout. Use combine_logs.py > outputfile
-to write to an outputfile."""
+to write to an outputfile.
+
+If no argument is provided, the most recent test directory will be used."""
 
 import argparse
 from collections import defaultdict, namedtuple
@@ -12,6 +14,13 @@
 import os
 import re
 import sys
+import tempfile
+
+# N.B.: don't import any local modules here - this script must remain executable
+# without the parent module installed.
+
+# Should match same symbol in `test_framework.test_framework`.
+TMPDIR_PREFIX = "bitcoin_func_test_"
 
 # Matches on the date format at the start of the log event
 TIMESTAMP_PATTERN = re.compile(
@@ -22,26 +31,36 @@
 
 def main():
     """Main function. Parses args, reads the log files and renders them as text or html."""
-
     parser = argparse.ArgumentParser(
-        usage='%(prog)s [options] <test temporary directory>', description=__doc__)
+        description=__doc__, formatter_class=argparse.RawTextHelpFormatter)
+    parser.add_argument(
+        'testdir', nargs='?', default='',
+        help='temporary test directory to combine logs from. '
+             'Defaults to the most recent')
     parser.add_argument('-c', '--color', dest='color', action='store_true',
-                        help='outputs the combined log with events colored by source (requires posix terminal colors. Use less -r for viewing)')
+                        help='outputs the combined log with events colored by '
+                             'source (requires posix terminal colors. Use less'
+                             ' -r for viewing)')
     parser.add_argument('--html', dest='html', action='store_true',
-                        help='outputs the combined log as html. Requires jinja2. pip install jinja2')
-    args, unknown_args = parser.parse_known_args()
+                        help='outputs the combined log as html. '
+                             'Requires jinja2. pip install jinja2')
+    args = parser.parse_args()
 
     if args.html and args.color:
         print("Only one out of --color or --html should be specified")
         sys.exit(1)
 
-    # There should only be one unknown argument - the path of the temporary
-    # test directory
-    if len(unknown_args) != 1:
-        print("Unexpected arguments" + str(unknown_args))
+    testdir = args.testdir or find_latest_test_dir()
+
+    if not testdir:
+        print("No test directories found")
         sys.exit(1)
 
-    log_events = read_logs(unknown_args[0])
+    if not args.testdir:
+        print("Opening latest test directory: {}".format(testdir),
+              file=sys.stderr)
+
+    log_events = read_logs(testdir)
 
     print_logs(log_events, color=args.color, html=args.html)
 
@@ -74,6 +93,27 @@ def read_logs(tmp_dir):
     return heapq.merge(*[get_log_events(source, f) for source, f in files])
 
 
+def find_latest_test_dir():
+    """Returns the latest tmpfile test directory prefix."""
+    tmpdir = tempfile.gettempdir()
+
+    def join_tmp(basename):
+        return os.path.join(tmpdir, basename)
+
+    def is_valid_test_tmpdir(basename):
+        fullpath = join_tmp(basename)
+        return (
+            os.path.isdir(fullpath)
+            and basename.startswith(TMPDIR_PREFIX)
+            and os.access(fullpath, os.R_OK)
+        )
+
+    testdir_paths = [join_tmp(name) for name in os.listdir(tmpdir)
+                     if is_valid_test_tmpdir(name)]
+
+    return max(testdir_paths, key=os.path.getmtime) if testdir_paths else None
+
+
 def get_log_events(source, logfile):
     """Generator function that returns individual log events.
 

test/functional/test_framework/test_framework.py

@@ -49,6 +49,8 @@ class TestStatus(Enum):
 # Timestamp is Dec. 1st, 2019 at 00:00:00
 TIMESTAMP_IN_THE_PAST = 1575158400
 
+TMPDIR_PREFIX = "bitcoin_func_test_"
+
 
 class SkipTest(Exception):
     """This exception is raised to skip a test"""
@@ -170,7 +172,7 @@ def main(self):
             self.options.tmpdir = os.path.abspath(self.options.tmpdir)
             os.makedirs(self.options.tmpdir, exist_ok=False)
         else:
-            self.options.tmpdir = tempfile.mkdtemp(prefix="test")
+            self.options.tmpdir = tempfile.mkdtemp(prefix=TMPDIR_PREFIX)
         self._start_logging()
 
         # Seed the PRNG. Note that test runs are reproducible if and only if