Move t/perl symlink creation back to the test_prep target of the Makefile

Move t/perl symlink creation back to the test_prep target of the Makefile

But not in the way it was done before with a recursive call to $(MAKE) -
instead create it directly in the current make invocation as an action for
test_prep, and create the link for perl.third directly as an action for
test_prep.third

Also, make it optional to pass the perl to run into runtests, by providing a
default of ./perl$(EXE_EXT)

This feels simpler and still Don't Repeat Yourself.