tests: Improve test coverage of C and Vala scanners
* xgettext-c-4: Check more trigraphs.
* xgettext-c-2: Check floating point literal.
* xgettext-c-19: Check out-of-range Unicode character, "\U"
character escape, incomplete "\u" escape, and surrogates.
* xgettext-vala-1: Check unterminated regexp literal, floating
point literal, string template, "/=" operator, multi-line
comments, and backslash-escaped character inside regexp literal.
