perf-lib: fix missing test titles in output

perf-lib: fix missing test titles in output

Commit 5dccd9155f (t/perf: add iteration setup mechanism to perf-lib,
2022-04-04) modified the parameter parsing of test_wrapper() such that
the test title was no longer in $1, and is instead in $test_title_.

We correctly pass the new variable to the code which outputs the title
to the log, but missed the spot in test_wrapper() where the title is
written to the ".descr" file which is used to produce the final output
table. As a result, all of the titles are missing from that table (or
worse, using whatever was left in $1):

  $ ./p0000-perf-lib-sanity.sh
  [...]
  Test           this tree
  ------------------------------
  0000.1:        0.01(0.01+0.00)
  0000.2:        0.01(0.00+0.01)
  0000.4:        0.00(0.00+0.00)
  0000.5: true   0.00(0.00+0.00)
  0000.7:        0.00(0.00+0.00)
  0000.8:        0.00(0.00+0.00)

After this patch, we get the pre-5dccd9155f output:

  Test                                                       this tree
  --------------------------------------------------------------------------
  0000.1: test_perf_default_repo works                       0.00(0.00+0.00)
  0000.2: test_checkout_worktree works                       0.01(0.00+0.01)
  0000.4: export a weird var                                 0.00(0.00+0.00)
  0000.5: éḿíẗ ńöń-ÁŚĆÍÍ ćḧáŕáćẗéŕś                          0.00(0.00+0.00)
  0000.7: important variables available in subshells         0.00(0.00+0.00)
  0000.8: test-lib-functions correctly loaded in subshells   0.00(0.00+0.00)

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>