py-compile tests: more faithful 'installcheck' support

py-compile tests: more faithful 'installcheck' support

* tests/py-compile-basedir.test: Use the `$am_scriptsdir' variable
instead of `$testsrcdir/../lib', to test more faithfully under
"make installcheck".  More uses of the `fatal_' subroutine to
diagnose hard errors.
* tests/py-compile-basic.test: Likewise.
* tests/py-compile-basic2.test: Likewise.
* tests/py-compile-destdir.test: Likewise.
* tests/py-compile-env.test: Likewise.
* tests/py-compile-option-terminate.test: Likewise.
* tests/py-compile-usage.test: Likewise.