Fix Python fun with strings vs. lists of strings.

Author: DougGregor

test/lit.cfg

@@ -2483,7 +2483,7 @@ config.substitutions.append(('%llvm-cov', config.llvm_cov))
 if hasattr(config, 'target_library_path_var'):
   host_library_env = (
           "/usr/bin/env " +
-          construct_library_path_env(config.swift_host_lib_dir))
+          construct_library_path_env([config.swift_host_lib_dir]))
 else:
   host_library_env = ""