NumberParser: for standard number format, try US locale first

For integers and fractional numbers using a decimal point, optionally
signed, we really want the result to be locale-independent.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Refactor the NumberFormatTest a bit

This should help point out the differences between the locales.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>