Add fr-BE as a test locale.

The latter uses non default weekdata which is useful for week-of-year/month
tests.

I've looked at replacing fr with fr-BE to keep the number of locales
constant but fails due to plurals data not having locale regions. This should
be fixed by unicode-org#834 which will also IIUC add all regional variants to testdata
anyways.

Author: mildgravitas

provider/testdata/Cargo.toml

@@ -66,7 +66,9 @@ locales = [
 
     # French:
     # - Often the first non-English locale to receive new data in CLDR
+    # - Belgian region has non default weekData.
     "fr",
+    "fr-BE",
 
     # Filipino:
     # - Week of month/year have plural variants.