perf tests: add "bindir" prefix to git tree test results

perf tests: add "bindir" prefix to git tree test results

Change the output file names in test-results/ to be
"test-results/bindir_<munged dir>" rather than just
"test-results/<munged dir>".

This is for consistency with the "build_" directories we have for
built revisions, i.e. "test-results/build_<SHA-1>".

There's no user-visible functional changes here, it just makes it
easier to see at a glance what "test-results" files are of what "type"
as they're all explicitly grouped together now, and to grep this code
to find both the run_dirs_helper() implementation and its
corresponding aggregate.perl code.

Note that we already guarantee that the rest of the
PERF_RESULTS_PREFIX is an absolute path, and since it'll start with
e.g. "/" which we munge to "_" we'll up with a readable string like
"bindir_home_avar[...]".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>