gimarshalingtests: Add tests for multiple utf8 args with allow-none

Add test function which accepts multiple allow-none utf8 arguments.
This can be used for testing a default of NULL for unsupplied
arguments annotated with allow-none.

https://bugzilla.gnome.org/show_bug.cgi?id=640812
gimarshalingtests: Add tests for mixed allow-none and implicit args

Add test method with an allow-none in the middle of non allow-none
arguments. Add test method with an array and its length argument
interleaved with allow-none arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=640812