Testsuite driver: Don’t do `/test spaces/` by default

The testsuite driver sets TEST_HC to a path with spaces to catch mistakes (unquoted $(TEST_HC) in Makefiles), as described in testsuite/mk/boilerplate.mk. This is laudable, but mildly annoying during local development; it makes already noisy log lines even noisier and longer and harder to select.

I suggest to make this optional, e.g. instead of or as part of

ifneq "$(wildcard $(STAGE1_TEST_SPACES))" ""

also check if some flag/variable is set that opts-into this behavior, and then set that flag in CI, but not by default. WDYT?