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

[ngie]

Summary

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: rXXXXXXX
Requested by: jhb

Test Plan

See "Test Plan" in D11905.