Export BAZEL_TEST=1 for bazel test executables

[mattyclarkson]

Catch2 has learnt how to output JUnit to the XML_OUTPUT_FILE. However, the upstream developers
felt that XML_OUTPUT_FILE is too generic to blanket enable JUnit output to the file as there are various
XML reporters for Catch2.

This patch enables exporting BAZEL_TEST environment variable so that Catch2 can enable JUnit output support
unconditionally in their build when both BAZEL_TEST and XML_OUTPUT_FILE are available.

[meteorcloudy]

It is possible to pass extra env var with --test_env, for example, --test_env=BAZEL=1. Can you please check if this works for you?

[mattyclarkson]

Can you please check if this works for you?

Well, yes, that would work.

However, do we expect anyone that uses {{catch2}} (or other test frameworks) to add {{--test_env}} to their {{.bazelrc}}. I would say that would be a UX/DX regression for usage of test frameworks. Unless we can propagate a test environment variable upon the usage of @catch2//:catch2_main, for example.

Bazel does test framework specific stuff in the test runner:

# Tell googletest about Bazel sharding.
if [[ -n "${TEST_TOTAL_SHARDS+x}" ]] && ((TEST_TOTAL_SHARDS != 0)); then
  export GTEST_SHARD_INDEX="${TEST_SHARD_INDEX}"
  export GTEST_TOTAL_SHARDS="${TEST_TOTAL_SHARDS}"
fi
export GTEST_TMP_DIR="${TEST_TMPDIR}"

# TODO(ulfjack): Update Gunit to accept XML_OUTPUT_FILE and drop this env
# variable.
GUNIT_OUTPUT="xml:${XML_OUTPUT_FILE}"

Removing GUNIT_OUTPUT would mean that GTest has to accept a very generic XML_OUTPUT_FILE, so seems like introducing a BAZEL=1 into the test runner would also allow GTest to check for BAZEL/XML_OUTPUT_FILE combo.

[meteorcloudy]

so seems like introducing a BAZEL=1 into the test runner would also allow GTest to check for BAZEL/XML_OUTPUT_FILE combo.

I see, seems like we should go with this option. But maybe choosing a more explicit name? Like BAZEL_TEST_SETUP?

[mattyclarkson]

Sure. Maybe BAZEL_TEST=1 as it's running under bazel test?

I'm not here for bikeshedding though and BAZEL_TEST_SETUP would be fine 😄

Depending on the naming I can respin the patch and PR description to match.

[meteorcloudy]

BAZEL_TEST sounds more clean, let's go with this. Please update this PR ;)

[meteorcloudy]

Thank you!

[meteorcloudy]

@sgowroji Please help import this PR once the tests pass ;)