Fix ApiErrorReporterTest, avoid use of $wgLang

Use a mock MessageParameterFormatter for instantiating
a new ExceptionLocalizer.

The formatter really isn't needed at all for what
the test is testing, so a mock is just as good or
better as it avoids use of $wgLang global.

Bug: 65372
Change-Id: Ia34b7d6af5da1344bce30bf63f0bb149f298e600
Merge "Fix ApiErrorReporterTest, avoid use of $wgLang"