Merge #29821: fuzz: Some test/fuzz/test_runner.py improvements

47cedee fuzz: Introduce `BITCOINFUZZ` environment variable (Hennadii Stepanov)
1573e9a fuzz, refactor: Deduplicate fuzz binary path creation (Hennadii Stepanov)

Pull request description:

  These changes are split from #29774 and can be beneficial on their own.

  The new `BITCOINFUZZ` environment variable complements the already existing set of variables used by tests: https://github.com/bitcoin/bitcoin/blob/b5d21182e5a66110ce2796c2c99da39c8ebf0d72/test/functional/test_framework/test_framework.py#L238-L243

ACKs for top commit:
  maflcko:
    lgtm ACK 47cedee
  davidgumberg:
    utACK 47cedee

Tree-SHA512: 45809cfd13dc4a45c44cc433184352e84726cb95bea80fd8f581c59a0b8b0a5495260ff66922f9c57c38adbdbdd102439238f370fd49d6ea27a241a5e6249895

test/fuzz/test_runner.py

@@ -104,9 +104,11 @@ def main():
         logging.error("Must have fuzz executable built")
         sys.exit(1)
 
+    fuzz_bin=os.getenv("BITCOINFUZZ", default=os.path.join(config["environment"]["BUILDDIR"], 'src', 'test', 'fuzz', 'fuzz'))
+
     # Build list of tests
     test_list_all = parse_test_list(
-        fuzz_bin=os.path.join(config["environment"]["BUILDDIR"], 'src', 'test', 'fuzz', 'fuzz'),
+        fuzz_bin=fuzz_bin,
         source_dir=config['environment']['SRCDIR'],
     )
 
@@ -151,7 +153,7 @@ def main():
     try:
         help_output = subprocess.run(
             args=[
-                os.path.join(config["environment"]["BUILDDIR"], 'src', 'test', 'fuzz', 'fuzz'),
+                fuzz_bin,
                 '-help=1',
             ],
             env=get_fuzz_env(target=test_list_selection[0], source_dir=config['environment']['SRCDIR']),
@@ -173,7 +175,7 @@ def main():
             return generate_corpus(
                 fuzz_pool=fuzz_pool,
                 src_dir=config['environment']['SRCDIR'],
-                build_dir=config["environment"]["BUILDDIR"],
+                fuzz_bin=fuzz_bin,
                 corpus_dir=args.corpus_dir,
                 targets=test_list_selection,
             )
@@ -184,7 +186,7 @@ def main():
                 corpus=args.corpus_dir,
                 test_list=test_list_selection,
                 src_dir=config['environment']['SRCDIR'],
-                build_dir=config["environment"]["BUILDDIR"],
+                fuzz_bin=fuzz_bin,
                 merge_dirs=[Path(m_dir) for m_dir in args.m_dir],
             )
             return
@@ -194,7 +196,7 @@ def main():
             corpus=args.corpus_dir,
             test_list=test_list_selection,
             src_dir=config['environment']['SRCDIR'],
-            build_dir=config["environment"]["BUILDDIR"],
+            fuzz_bin=fuzz_bin,
             using_libfuzzer=using_libfuzzer,
             use_valgrind=args.valgrind,
             empty_min_time=args.empty_min_time,
@@ -237,7 +239,7 @@ def transform_rpc_target(targets, src_dir):
     return targets
 
 
-def generate_corpus(*, fuzz_pool, src_dir, build_dir, corpus_dir, targets):
+def generate_corpus(*, fuzz_pool, src_dir, fuzz_bin, corpus_dir, targets):
     """Generates new corpus.
 
     Run {targets} without input, and outputs the generated corpus to
@@ -270,7 +272,7 @@ def job(command, t, t_env):
         os.makedirs(target_corpus_dir, exist_ok=True)
         use_value_profile = int(random.random() < .3)
         command = [
-            os.path.join(build_dir, 'src', 'test', 'fuzz', 'fuzz'),
+            fuzz_bin,
             "-rss_limit_mb=8000",
             "-max_total_time=6000",
             "-reload=0",
@@ -283,12 +285,12 @@ def job(command, t, t_env):
         future.result()
 
 
-def merge_inputs(*, fuzz_pool, corpus, test_list, src_dir, build_dir, merge_dirs):
+def merge_inputs(*, fuzz_pool, corpus, test_list, src_dir, fuzz_bin, merge_dirs):
     logging.info(f"Merge the inputs from the passed dir into the corpus_dir. Passed dirs {merge_dirs}")
     jobs = []
     for t in test_list:
         args = [
-            os.path.join(build_dir, 'src', 'test', 'fuzz', 'fuzz'),
+            fuzz_bin,
             '-rss_limit_mb=8000',
             '-set_cover_merge=1',
             # set_cover_merge is used instead of -merge=1 to reduce the overall
@@ -325,13 +327,13 @@ def job(t, args):
         future.result()
 
 
-def run_once(*, fuzz_pool, corpus, test_list, src_dir, build_dir, using_libfuzzer, use_valgrind, empty_min_time):
+def run_once(*, fuzz_pool, corpus, test_list, src_dir, fuzz_bin, using_libfuzzer, use_valgrind, empty_min_time):
     jobs = []
     for t in test_list:
         corpus_path = corpus / t
         os.makedirs(corpus_path, exist_ok=True)
         args = [
-            os.path.join(build_dir, 'src', 'test', 'fuzz', 'fuzz'),
+            fuzz_bin,
         ]
         empty_dir = not any(corpus_path.iterdir())
         if using_libfuzzer: