Export BAZEL_XML_OUTPUT_FILE

[Catch2][catch2] has [learnt][pr2399] how to output JUnit to the `XML_OUTPUT_FILE`. However, the upstream developers
[felt][comment] 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 prefixing the `XML_OUTPUT_FILE` environment variable with `BAZEL_` so that Catch2 can enable JUnit
output support unconditionally in their build.

[catch2]: https://github.com/catchorg/Catch2
[pr2399]: catchorg/Catch2#2399
[comment]: catchorg/Catch2#2399 (comment)

tools/test/test-setup.sh

@@ -68,6 +68,10 @@ is_absolute "$TEST_TMPDIR" || TEST_TMPDIR="$PWD/$TEST_TMPDIR"
 is_absolute "$HOME" || HOME="$TEST_TMPDIR"
 is_absolute "$XML_OUTPUT_FILE" || XML_OUTPUT_FILE="$PWD/$XML_OUTPUT_FILE"
 
+# Enable JUnit output in test frameworks that need to detect Bazel
+BAZEL_XML_OUTPUT_FILE="$XML_OUTPUT_FILE"
+export BAZEL_XML_OUTPUT_FILE
+
 # Set USER to the current user, unless passed by Bazel via --test_env.
 if [[ -z "$USER" ]]; then
   export USER=$(whoami)