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

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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.

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

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    Fix a warning in the Azure Pipelines runs
    
    Yet another thing I stumbled across...

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-552%2Fdscho%2Fwrite-junit-xml-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-552/dscho/write-junit-xml-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/552

 t/test-lib.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)