Add tests for filename_language

Add tests for filename_language

The next patch touches the filename_language area, but I noticed there
is no test exercising that.  This patch adds some selftests for
add_filename_language, deduce_language_from_filename and
set_ext_lang_command.  Because these tests add entries to the global
filename_language_table vector, it is not possible to run them
successfully multiple times in a same GDB instance.  They can
potentially interfere with each other for the same reason.  I therefore
added the scoped_restore_filename_language_table class that is used to
make sure tests leave that global vector in the same state they found it
(it is replaced in the following patch by a simple scoped_restore).

gdb/ChangeLog:

	* symfile.c: Include selftest.h.
	(class scoped_restore_filename_language_table): New.
	(test_filename_language): New test.
	(test_set_ext_lang_command): New test.
	(_initialize_symfile): Register tests.