Recently we have declared that GIT_TEST_* variables take the
usual boolean values (it used to be that some used "non-empty
means true" and taking GIT_TEST_VAR=YesPlease as true); make
sure we notice and fail when non-bool strings are given to
these variables.
* sg/test-bool-env:
t5608-clone-2gb.sh: turn GIT_TEST_CLONE_2GB into a bool
tests: add 'test_bool_env' to catch non-bool GIT_TEST_* values
e0f9ec9027