gstrutils: Set locale explicitly for search-utils test

Merged nightuser requested to merge nightuser/glib:tests_search-utils_locale_fix into master

The test checks g_str_match_string() function, which performs matches based on the user's locale. For this reason, some tests may fail, e.g., see issue #868 (closed).

Now we explicitly set locale for each test, with C locale as a fallback when the locale is not available.

Currently, I added fr_FR, sv_SE, and tr_TR locales for the tests with Unicode symbols, and C for everything else. In case the requested locale is not available, it is reported via INFO-message (it's not a warning, because it's not critical for the test). Due to this change is might be needed to make sure that this locales are available on the CI images.

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