Re: [PATCH] tests: fix --write-junit-xml with subshells

"Johannes Schindelin via GitGitGadget" [off-list ref]
writes:

From: Johannes Schindelin <redacted>

In t0000, more precisely in its `test_bool_env` test case, there are two
subshells that are supposed to fail. To be even _more_ precise, they
fail by calling the `error` function, and that is okay, because it is in
a subshell, and it is expected that those two subshell invocations fail.

However, the `error` function also tries to finalize the JUnit XML (if
that XML was asked for, via `--write-junit-xml`. As a consequence, the
XML is edited to add a `time` attribute for the `testsuite` tag. And
since there are two expected `error` calls in addition to the final
`test_done`, the `finalize_junit_xml` function is called three times and
naturally the `time` attribute is added _three times_.

Azure Pipelines is not happy with that, complaining thusly:

 ##[warning]Failed to read D:\a\1\s\t\out\TEST-t0000-basic.xml. Error : 'time' is a duplicate attribute name. Line 2, position 82..

One possible way to address this would be to unset `write_junit_xml` in
the `test_bool_env` test case.

But that would be fragile, as other `error` calls in subshells could be
introduced.

Interesting.  Naïvely, I would have expected that the right fix,
especially after reading "as other error calls can be introduced"
was to stop calling finalize from error() and instead call it from
the exit trap, but that would not work, as the exit trap would also
be called even from a subshell X-<.

So let's just modify `finalize_junit_xml` to remove any `time` attribute
before adding the authoritative one.

It feels wrong to repeatedly rewrite the $junit_xml_path file during
the test, and it feels doubly wrong to call finalize many number of
times, but given that we do not have a good way to revamp the code
to stop doing so, I think "strip existing time attribute, if any,
and then add what we have" we see here is the best solution
available.

Will queue.  Thanks.