Avoid unnecessary rebuilds of test targets

[mhx]

Currently, all targets created by pytest_discover_tests will cause a rebuild every time make / ninja is called, even if no files have changed since the last build.

Quoting the CMake documentation for add_custom_target:

The target has no output file and is always considered out of
date even if the commands try to create a file with the name of
the target. Use the add_custom_command() command to generate a
file with dependencies.

This change should be functionally equivalent, but avoids the indefinite rebuilds because the target will only check for the presence of ${_tests_file} rather than unconditionally building it.

[buddly27]

That's a great update, thanks for contributing!