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

diff --cc t/README
Simple merge

diff --cc t/t0000-basic.sh
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge

diff --cc t/test-lib.sh
Simple merge