Fix problems with a locale-dependent decimal mark in StringContextTest

This commit corrects three tests which were failing for certain locale
due to the different decimal marks in the expected value and the
result (e.g. 2.50 and 2,50).

From now also the expected value is formatted in accordance with the
current locale.

I reported this issue some time ago (see
https://groups.google.com/forum/#!topic/scala-internals/P7mv3wmTSfs )
but nothing changed so far. That's why I made these changes.