Add environment var support to runtime tests

Some `bpftrace` behaviour can only be modified by environment vars. To
make that testable the test suite needs to allow for env vars.

It supports both multiple `key=value` options on a single line and
multiple `ENV` lines, merging them in the order in which they're read.
So:

```
RUN xxx
ENV X=1 Y=2
```

and

```
RUN xxx
ENV X=1
ENV Y=2
```

Are identical.

tests/parser.py

@@ -15,7 +15,7 @@ class UnknownFieldError(Exception):
     pass
 
 
-TestStruct = namedtuple('TestStruct', 'name run expect timeout before after suite kernel requirement')
+TestStruct = namedtuple('TestStruct', 'name run expect timeout before after suite kernel requirement env')
 
 
 class TestParser(object):
@@ -64,6 +64,7 @@ def __read_test_struct(test, test_suite):
         after = ''
         kernel = ''
         requirement = ''
+        env = {}
 
         for item in test:
             item_split = item.split()
@@ -86,6 +87,10 @@ def __read_test_struct(test, test_suite):
                 kernel = line
             elif item_name == 'REQUIRES':
                 requirement = line
+            elif item_name == 'ENV':
+                for e in line.split():
+                    k, v = e.split('=')
+                    env[k]=v
             else:
                 raise UnknownFieldError('Field %s is unknown. Suite: %s' % (item_name, test_suite))
 
@@ -98,4 +103,4 @@ def __read_test_struct(test, test_suite):
         elif timeout == '':
             raise RequiredFieldError('Test TIMEOUT is required. Suite: ' + test_suite)
 
-        return TestStruct(name, run, expect, timeout, before, after, test_suite, kernel, requirement)
+        return TestStruct(name, run, expect, timeout, before, after, test_suite, kernel, requirement, env)

tests/utils.py

@@ -88,12 +88,14 @@ def run_test(test):
                 before = subprocess.Popen(test.before, shell=True)
 
             bpf_call = Utils.prepare_bpf_call(test)
+            env = {'test': test.name}
+            env.update(test.env)
             p = subprocess.Popen(
                 bpf_call,
                 shell=True,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.STDOUT,
-                env={"test": test.name},
+                env=env,
                 bufsize=1
             )