Merge pull request #2723 from akoeplinger/fix-decimal-test-culture

[corlib] Fix three decimal tests to not rely on current culture for formatting

They'd fail on cultures where "." is not the decimal separator before.

I've also reenabled them on mobile cause the workaround for #27269 is no longer necessary since all bots are now on Mono 4.0+.

@monojenkins merge