Make the TestSettings.py test cases use different output filenames:

o "output1.txt" for test_pass_host_env_vars() test case
o "output2.txt" for test_run_args_and_env_vars_with_dsym() test case
o "output2.txt" for test_run_args_and_env_vars_with_dwarf() test case

and add teardown hook to test_pass_host_env_vars() in order to properly
unset the host environment variables set while running the test case.

llvm-svn: 121811