tests: allow to run Perl tests more cleanly

tests: allow to run Perl tests more cleanly

Before this change, perl tests were run via a #!/bin/sh script
in which perl was invoked via $(PERL) ... -- - <<\EOF.
That made some stty tests fail due to the way stdin was usurped.
* build-aux/check.mk (am__check_pre): Don't append $(SHELL) to this
nominally automake-internal variable.
* tests/check.mk (TESTS_ENVIRONMENT): Instead, define a shell function
here, and append it to the more user-visible $(TESTS_ENVIRONMENT).