unit-tests: use clean test environment

So far, unit-tests are run in the user's environment, in particular with
the user's git config which can influence test runs.

Set up the same clean test environment which we use for the other tests.

Author: mjg

t/Makefile

@@ -83,7 +83,7 @@ prove: pre-clean $(TEST_LINT)
 	$(MAKE) clean-except-prove-cache
 
 $(T):
-	@echo "*** $@ ***"; '$(TEST_SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
+	@echo "*** $@ ***"; '$(TEST_SHELL_PATH_SQ)' ./run-test.sh $@ $(GIT_TEST_OPTS)
 
 $(UNIT_TESTS):
 	@echo "*** $@ ***"; $@

t/run-test.sh

@@ -13,6 +13,7 @@ case "$1" in
 	exec "${TEST_SHELL_PATH}" "$@" ${TEST_OPTIONS}
 	;;
 *)
+	. ./unittest-lib.sh
 	exec "$@"
 	;;
 esac