test-lib: have --immediate emit valid TAP on failure

test-lib: have --immediate emit valid TAP on failure

Change the "--immediate" option so that it emits valid TAP on
failure. Before this it would omit the required plan at the end,
e.g. under SANITIZE=leak we'd show a "No plan found in TAP output"
error from "prove":

    $ prove t0006-date.sh ::  --immediate
    t0006-date.sh .. Dubious, test returned 1 (wstat 256, 0x100)
    Failed 1/22 subtests

    Test Summary Report
    -------------------
    t0006-date.sh (Wstat: 256 Tests: 22 Failed: 1)
      Failed test:  22
      Non-zero exit status: 1
      Parse errors: No plan found in TAP output
    Files=1, Tests=22,  0 wallclock secs ( 0.02 usr  0.01 sys +  0.18 cusr  0.06 csys =  0.27 CPU)
    Result: FAIL

Now we'll emit output that doesn't result in TAP parsing failures:

    $ prove t0006-date.sh ::  --immediate
    t0006-date.sh .. Dubious, test returned 1 (wstat 256, 0x100)
    Failed 1/22 subtests

    Test Summary Report
    -------------------
    t0006-date.sh (Wstat: 256 Tests: 22 Failed: 1)
      Failed test:  22
      Non-zero exit status: 1
    Files=1, Tests=22,  0 wallclock secs ( 0.02 usr  0.00 sys +  0.19 cusr  0.05 csys =  0.26 CPU)
    Result: FAIL

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>