tests: add few missing 'cc' and 'c++' requirements

tests: add few missing 'cc' and 'c++' requirements

Without that requirements, the configure-time overrides of CC and CXX
wouldn't be honored in the affected tests.

* t/silent-yacc.sh ($required): Require 'cc'.
* t/silent-yacc-headers.sh: Likewise.
* t/silent-lex.sh: Likewise.
* t/lex-clean.sh: Likewise.
* t/lex-depend.sh: Likewise.
* t/lex-line.sh: Likewise.
* t/lex-depend.sh: Likewise.
* t/lex-pr204.sh: Likewise.
* t/yacc-deleted-headers.sh: Likewise.
* t/yacc-line.sh: Likewise.
* t/yacc-depend.sh: Likewise.
* t/yacc-depend2.sh: Likewise.
* t/yacc-dist-nobuild-subdir.sh: Likewise.
* t/yacc-bison-skeleton.sh: Likewise.
* t/yacc-bison-skeleton-cxx.sh ($required): Require 'c++'.
* t/yacc-clean-cxx.sh: Likewise.
* t/yacc-d-cxx.sh: Likewise.
* t/yacc-cxx.sh: Likewise.
* t/yacc-mix-c-cxx.sh ($required): Require 'cc' and 'c++'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>