ci: avoid running the test suite _twice_

This is a late amendment of 4a6e4b9 (CI: remove Travis CI support,
2021-11-23), whereby the `.prove` file (being written by the `prove`
command that is used to run the test suite) is no longer retained
between CI builds: This feature was only ever used in the Travis CI
builds, we tried for a while to do the same in Azure Pipelines CI runs
(but I gave up on it after a while), and we never used that feature in
GitHub Actions (nor does the new GitLab CI code use it).

Retaining the Prove cache has been fragile from the start, even though
the idea seemed good at the time, the idea being that the `.prove` file
caches information about previous `prove` runs (`save`) and uses them
(`slow`) to run the tests in the order from longer-running to shorter
ones, making optimal use of the parallelism implied by `--jobs=<N>`.

However, using a Prove cache can cause some surprising behavior: When
the `prove` caches information about a test script it has run,
subsequent `prove` runs (with `--state=slow`) will run the same test
script again even if said script is not specified on the `prove`
command-line!

So far, this bug did not matter. Right until d8f416b (ci: run unit
tests in CI, 2023-11-09) did it not matter.

But starting with that commit, we invoke `prove` _twice_ in CI, once to
run the regular test suite of regression test scripts, and once to run
the unit tests. Due to the bug, the second invocation re-runs all of the
tests that were already run as part of the first invocation. This not
only wastes build minutes, it also frequently causes the `osx-*` jobs to
fail because they already take a long time and now are likely to run
into a timeout.

The worst part about it is that there is actually no benefit to keep
running with `--state=slow,save`, ever since we decided no longer to
try to reuse the Prove cache between CI runs.

So let's just drop that Prove option and live happily ever after.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

ci/lib.sh

@@ -281,7 +281,7 @@ else
 fi
 
 MAKEFLAGS="$MAKEFLAGS --jobs=$JOBS"
-GIT_PROVE_OPTS="--timer --jobs $JOBS --state=failed,slow,save"
+GIT_PROVE_OPTS="--timer --jobs $JOBS"
 
 GIT_TEST_OPTS="$GIT_TEST_OPTS --verbose-log -x"
 case "$CI_OS_NAME" in