Add supporting changes for `Add limited sandbox capability to "make check"`

Add supporting changes for `Add limited sandbox capability to "make check"`

Non-tests/... changes:
- Add HAS_TESTS= to Makefiles with libraries and programs to enable iteration
  and propagate the appropriate environment down to *.test.mk.

tests/... changes:
- Add appropriate support Makefile.inc's to set HAS_TESTS in a minimal manner,
  since tests/... is a special subdirectory tree compared to the others.

MFC after:	2 months
MFC with:	r322511
Reviewed by:	arch (silence), testing (silence)
Differential Revision:	D12014