[tests] Add mechanism to override lit environment

lit runs its tests with only the environment variables set up by its configuration, which is great for repeatability, but sometimes you want to allow a few controlled leaks. With this change, you can now specify “--param copy_env=FOO:BAR:BAZ” to cause the environment variables FOO, BAR, and BAZ to be visible in lit tests during that run.

Author: beccadax

test/lit.cfg

@@ -1665,5 +1665,12 @@ if platform.system() == 'Linux':
         config.available_features.add("LinuxDistribution=" + distributor + '-' + release)
         lit_config.note('Running tests on %s-%s' % (distributor, release))
 
+# Copy environment variables specified in --param copy_env=..., overriding
+# anything computed here. It's assumed that the person setting this knows what
+# they're doing.
+copy_env = lit_config.params.get('copy_env', None)
+if copy_env is not None:
+  for key in copy_env.split(':'):
+    config.environment[key] = os.environ[key]
 
 lit_config.note("Available features: " + ", ".join(sorted(config.available_features)))