tests: Don't compare strings by pointer

Merged tbaederr requested to merge tbaederr/glib:wip/tbaederr/clang-warnings into master

clang complains about this in the form of

:6:9: warning: result of comparison against a string literal is unspecified (use an explicit string comparison function instead)

if (f == (void *)"a") { ^ ~~~~~~~~~~~

Use variables for the strings instead, which should have the same address.

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