TestSuite: added missing include to TesterTest.

GCC 4.7.2 on MinGW32 on Windows complained.
Windows build fixes.
TestSuite: minor test reorganization.
TestSuite: properly test CORRADE_COMPARE_AS().

Currently the compilation fails on ambiguous overload.
TestSuite: fix ambiguous overload for CORRADE_COMPARE_AS().

Test now builds and passes.