Fix spurious silent*.test failures for $CC != gcc

Fix spurious silent*.test failures for $CC != gcc

In some tests on automake-produced silent rules, we forced the
use of gcc depmode to improve testsuite coverage; but this has
unsurprisingly led to spurious failures when some non-GNU C
compilers were used.  So we are now careful to require GCC in
tests that force gcc depmode.

From reports by Ralf Wildenhues.

* silent5.test: Test removed, its content split into ...
* silent-many-generic.test, silent-many-gcc.test: ... these new
sister tests, the latter of which forces gcc depmode and lists
"gcc" in $required.
* silentlex.test: Test removed, its content split into ...
* silent-lex-generic.test, silent-lex-gcc.test: ... these new
sister tests, the latter of which forces gcc depmode and lists
"gcc" in $required.
* silentyacc.test: Test removed, its content split into ...
* silent-yacc-generic.test, silent-yacc-gcc.test: ... these new
sister tests, the latter of which forces gcc depmode and lists
"gcc" in $required.
* tests/Makefile.am (TESTS): Updated.