Merge branch 'sg/test-bool-env'

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