compiletest: Add a variant of `{{build-base}}` that points to the current test's build subdirectory

[Zalathar]

Several of the existing tests that use {{build-base}} would be better off using a test-specific output subdirectory (or an automatically-cleared scratch directory), rather than the “root” output directory shared by all tests in a test suite.

Unfortunately, this is currently hard to accomplish because parse_name_value_directive doesn't have access to any test-specific data structures, only the &Config shared by all tests in the suite.

[Zalathar]

This was originally motivated by #147217, but then I realised that we have a dedicated //@ incremental directive to handle that particular scenario.