I searched on https://github.com/bazelbuild/bazel/blob/4e43b4aaa3a0f85a6390326e245c675c091c965b/tools/test/windows/tw_test.cc to see how could I test this, but those are not blackbox run a subprocess and see what env variables were actually sent type of tests.

Maybe that test is somewhere but I couldn't find it. If you could point me to it, I could perhaps add the test?

You could add an assertion to https://github.com/bazelbuild/bazel/blob/master/src/test/py/bazel/test_wrapper_test.py or a new test to https://github.com/bazelbuild/bazel/blob/master/src/test/py/bazel/bazel_windows_test.py.

Sorry for the messy CLs, but I don't want to install Java (and the other toolchains that might be required) just for this, so I'm abusing the CI a bit to run the tests for me.
I didn't find info about Bazel distributing the toolchains required. Perhaps if there is a way you could point me to it?

Okay I stopped being lazy and figured how to run the tests locally (--java_runtime_version=remotejdk_11).
Now it has an actual test verifying that we're supplying the BAZEL_TEST env variable.

Sorry for the spam.