Updated test/lit.cfg to respect the TOOLCHAINS environment variable when deciding the xcrun toolchain for Darwin platforms, rather than hardcoding the default.

Author: Rostepher

test/lit.cfg

@@ -152,10 +152,11 @@ if config.test_exec_root is None:
 # name: The name of this test suite.
 config.name = 'Swift(%s)' % config.variant_suffix[1:]
 
-# Tweak the environment appropriately for various platforms.
+# Respect the TOOLCHAINS environment variable when deciding the xcrun
+# toolchain for Darwin platforms.
 if platform.system() == 'Darwin':
-    # Prefer the latest version of the Xcode tools.
-    config.environment['TOOLCHAINS'] = 'default'
+    config.environment['TOOLCHAINS'] = \
+        os.environ.get('TOOLCHAINS', config.darwin_xcrun_toolchain)
 
 # testFormat: The test format to use to interpret tests.
 config.test_format = swift_test.SwiftTest(coverage_mode=config.coverage_mode)