Test that match against error numbers, dates should set en_US locale, ot...

...herwise error messages and date literals do not match

Several tests compare error messages here and there. When the locale is not en_US, number and date formats might differ.

On my machine `1000` is formatted as `1 000`, while tests expect `1,000`.

I've added `@BeforeTest Locale.setDefaultLocale` so the test can be run in both maven and IDE (works in IDEA)