Fix unnecessary nesting in run-make test output directories
Run-make tests were using `output_base_name` to determine their output directory, which results in a redundant subdirectory (e.g. `$build/test/run-make/<foo>/<foo>/`) because that method is intended to produce the name of an individual file.
The previous attempt to fix this double-nesting tried adding a special case in `output_base_dir`, which had the side-effect of breaking up-to-date checking for run-make tests, and had to be reverted in #131681.
The fix is simply to call `output_base_dir` directory, which gives the desired directory without any redundant part.
r? jieyouxu
43a142e4d0