tests: remove redundancy from silent lex/yacc tests

tests: remove redundancy from silent lex/yacc tests

* tests/silent-yacc-gcc.test, tests/silent-yacc-generic.test: Merge
these two testcases into ...
* tests/silent-yacc.test: ... this new one, which doesn't fiddle
which dependency tracking, as that shouldn't impact on generation
of yacc-derived C source and header files (and, for what concerns
compilation of C files, is already tested in other testcases).
* tests/silent-lex-gcc.test, tests/silent-lex-generic.test: Merge
these two testcases into ...
* tests/silent-lex.test: ... this new test, for similar reasons.
* tests/Makefile.am (TESTS): Update.