Rename BUILDMASTER_TEST to BUILDBOT_TEST (llvm#301)

BUILDBOT_TEST is sufficiently descriptive (after all, 'buildbot' is the
package name and the name of the binary invoked to create a buildmaster)
and it's possible upstream may later remove the buildmaster terminology
<buildbot/buildbot#5382> or alternatively we
might move away from it downstream. As such, it seems prudent to use a
name that would be unaffected.

BUILDMASTER_TEST was added in llvm#289 and as the docs PR hasn't landed yet, shouldn't have any users relying on it yet.

buildbot/osuosl/master/master.cfg

@@ -26,10 +26,10 @@ c['buildbotNetUsageData'] = None
 import config
 reload(config)
 
-# Detect if the BUILDMASTER_TEST environment variable is set and non-zero.
-# This will be used to enable a local testing mode.
-buildmaster_test = os.environ.get('BUILDMASTER_TEST')
-test_mode = bool(buildmaster_test) and buildmaster_test != '0'
+# Detect if the BUILDBOT_TEST environment variable is set and non-zero. This
+# will be used to enable a local testing mode.
+buildbot_test = os.environ.get('BUILDBOT_TEST')
+test_mode = bool(buildbot_test) and buildbot_test != '0'
 config.options['Internal'] = {}
 config.options['Internal']['test_mode'] = str(test_mode)