language-specs: fix failing tests on win32
I ran today the tests under Windows10 + msys2 and got the following test failure:
not ok /LanguageManager/guess-language - GtkSourceView-FATAL-ERROR: Expected NULL language, got yaml
from this code:
l = gtk_source_language_manager_guess_language (lm, "foo.abcdef", "");
assert_null_language (l);
It appears that "" matches the languages that have a trailing semicolons at the end of their mimetypes list, so I trimmed them in the language-specs.
Also, the content type descendant test fails on win32 as the underlying Gio function doesn't use mimetype database, so I disabled it.