perf-lib: fix start/stop of perf tests

ae75342 test-lib: rearrange start/end of test_expect_* and test_skip
changed the way tests are started/stopped, but did not update the perf
tests.  They were therefore giving the wrong output, because of the
wrong test count.  Fix this by starting and stopping the tests
correctly.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Acked-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>