tests: more coverage on yacc/lex silent-rules, plus minor cleanups

tests: more coverage on yacc/lex silent-rules, plus minor cleanups

* tests/silent-yacc-gcc.test: Add sanity checks verifying that the
generated Makefile.in files really contains the non-generic rules
we expect.  Do not redundantly manually remove files we know to be
already removed "make maintainer-clean".
(Makefile.am): Ensure we cover also non-generic yacc rules, by
setting target-specific YFLAGS.
(sub/Makefile.am): Likewise.
* tests/silent-yacc-generic.test: Likewise.
* tests/silent-lex-gcc.test: Likewise, but with LFLAGS instead of
YFLAGS.
* tests/silent-lex-generic.test: Likewise.
* tests/silent-many-gcc.test: Likewise, but with both LFLAGS and
YFLAGS.  Also ...
(do_and_check_verbose_build): Remove redundant blank line.
* tests/silent-many-generic.test: Likewise.
* tests/silent-yacc-headers.test: New test.
* tests/Makefile.am (TESTS): Update.