[lldb] Set the TERM environment variable for the API tests

Avoid "TERM environment variable not set" by either propagating the TERM
environment variable or defaulting to vt100. All of our CI is already
doing this explicitly through the --env dotest arg, but it's easy to
forget when setting up a new job. I don't see any downside in making it
the default.

lldb/test/API/lit.cfg.py

@@ -235,6 +235,9 @@ import lldbtest
 # testFormat: The test format to use to interpret tests.
 config.test_format = lldbtest.LLDBTest(dotest_cmd)
 
+# Propagate TERM or default to vt100.
+config.environment['TERM'] = os.getenv('TERM', default='vt100')
+
 # Propagate FREEBSD_LEGACY_PLUGIN
 if 'FREEBSD_LEGACY_PLUGIN' in os.environ:
   config.environment['FREEBSD_LEGACY_PLUGIN'] = os.environ[