t/perf/perf-lib.sh: remove test_times.* at the end test_perf_()

t/perf/perf-lib.sh: remove test_times.* at the end test_perf_()

Teach test_perf_() to remove the temporary test_times.* files
at the end of each test.

test_perf_() runs a particular GIT_PERF_REPEAT_COUNT times and creates
./test_times.[123...].  It then uses a perl script to find the minimum
over "./test_times.*" (note the wildcard) and writes that time to
"test-results/<testname>.<testnumber>.result".

If the repeat count is changed during the pXXXX test script, stale
test_times.* files (from previous steps) may be included in the min()
computation.  For example:

...
GIT_PERF_REPEAT_COUNT=3 \
test_perf "status" "
	git status
"

GIT_PERF_REPEAT_COUNT=1 \
test_perf "checkout other" "
	git checkout other
"
...

The time reported in the summary for "XXXX.2 checkout other" would
be "min( checkout[1], status[2], status[3] )".

We prevent that error by removing the test_times.* files at the end of
each test.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>