Fixed the chruby_auto tests that were failing on GitHub Action's macOS.

* GitHub Action's macOS image provides a different version of shunit2.
* shnit2 runs tearDown an additional time after all test_* functions
  have been called.
  kward/shunit2#112

test/chruby_auto_test.sh

@@ -2,6 +2,7 @@
 . ./test/helper.sh
 
 test_dir="$test_fixtures_dir/ruby_versions"
+old_pwd="$PWD"
 
 function oneTimeSetUp()
 {
@@ -165,7 +166,7 @@ function test_chruby_auto_ruby_version_containing_options()
 
 function tearDown()
 {
-	cd "$PWD"
+	cd "$old_pwd"
 }
 
 function oneTimeTearDown()