Throw an error when reference files are missing.

[fruffy]

Fixes #2892.

With this pull request, tests will fail if they are missing their reference files.

[fruffy]

The one problem with this is that it is not easy to generate reference files for many tests at once or even single tests.
The question is: What kind of workflow should be used to actually generate reference files? Do we have an argument for ctest/make check? Or do I need to add one? It should probably be documented somewhere, too.

[mihaibudiu]

A good rule of thumb is to run make check and to commit all files under ./testsuite/.
Note: if the environment variable P4TEST_REPLACE is set the tests will always overwrite their reference outputs.

[fruffy]

I added this message to help users in the case of an error:
"Missing reference for file %s. Please rerun the test with the -f option turned on or rerun all tests using \"P4TEST_REPLACE=True make check\"."

This check is working now, and it already caught a couple tests that were missing their reference files.