DemangleTest: Add a test for demangle_symbol_gcc3 without the full demangler.

This tests what 0945c7e4e6b52126886ef81d8f217690f0dbe7e6 fixed.
Also fix one of the GCC2 tests which had expectation and input swapped.

demangler/gcc3+: handle vendor-specific suffix

fix bug #13720

Change-Id: I3363572b88aaa674f5ee7cf69866b9f2bbb4e84c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4084
Reviewed-by: Rene Gollent <rene@gollent.com>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

demangle/gcc2: Skip count > length should skip to the end.

This is the behavior the code expects.

tests: Add some more things to DemangleTest.

Change-Id: I6999aad2cc64650bddb7ba575536efc8878abcf2
Reviewed-on: https://review.haiku-os.org/583
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

tests: Add test boilerplate for our homegrown demanglers.

There's only one test each, for now, but adding tests is now of course
a one-line addition per test.