make Get-TestTime culture agnostic

On cs-CZ, de-DE and other systems where decimal separator is different
than ".". Value [double]3.5 is output as 3,5 (or similar). Currently
this only makes some of the tests fail, but it could also leak to the
nUnit report if the time was output.
Merge pull request #111 from nohwnd/TestResultCultureAgnostic

make Get-TestTime culture agnostic