tests: avoid gross inefficiency in "make test"

tests: avoid gross inefficiency in "make test"

Do not run a sub-make to set up the environment for each
and every test script.  Instead, run it just once and store
the result in a file.
* tests/check.mk (built_programs): Remove definition.
(.built-programs): New rule to create the temporary file.
(CLEANFILES): Arrange to remove it.
(TESTS_ENVIRONMENT): Simply cat .built-programs, rather than
running the sub-make.
* .gitignore: Ignore it.