Speculative fix for test output race conditions

Sometimes two tests will simultaneously try to write the same file in the temporary directory. 

See rdar://31520207

Author: Dave Abrahams

test/lit.cfg

@@ -357,6 +357,11 @@ completion_cache_path = tempfile.mkdtemp(prefix="swift-testsuite-completion-cach
 ccp_opt = "-completion-cache-path %r" % completion_cache_path
 lit_config.note("Using code completion cache: " + completion_cache_path)
 
+# Apparently despite the docs, tmpDir is not actually unique for each test, but
+# tmpBase is.  Speculative fix for rdar://31520207.
+config.substitutions.remove( ('%T', tmpDir) )
+config.substitutions.append( ('%T', tmpBase) )
+
 config.substitutions.append( ('%swift_obj_root', config.swift_obj_root) )
 config.substitutions.append( ('%swift_src_root', config.swift_src_root) )
 config.substitutions.append( ('%{python}', sys.executable) )