CI: Collect logs of failed unit tests (#2348)

CI: Collect logs of failed unit tests (#2348)

When a unit test fails during CI checks, the corresponding GitHub
Actions reports and collected btlayer-*.log files do not contain
any failure details. For example, we see

    FAIL: tests/testRock
    # FAIL: 1

but are missing critical details like

    stub time| FATAL: Ipc::Mem::Segment::create failed to
    shm_open(/squid-0-tr_rebuild_versions.shm): (63) File name too long

Now, GitHub Actions collect all log files, including unit test logs. For
the `ubuntu-24.04,gcc,default` build target, adding more logs increases
artifacts zip archive size by about 100 KB (from ~200KB to ~300KB).

If `test-builds.sh` succeeds, there are no unit test logs to collect
because all unit test logs are erased when `make distcheck` (initiated
by `test-builds.sh`) reaches its `make distclean` step. If a unit test
fails, then that cleaning step is not reached, (successful and failed)
unit test logs are preserved and are now added to CI artifacts.