gstvalue: only unwrap string delimited with "

gstvalue: only unwrap string delimited with "

Don't unwrap strings that start but don't finish with a double quote. If a
string is delimited by two quotes we unescape them and any special characters
in the middle (like \" or \\). If the first character or the last character
aren't a quote we assume it's part of an unescaped string.

Moved some deserialize_string unit tests because we don't try to unwrap strings
missing that second quote anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=688625