Enhance test paths

If the test paths are changed by an including project, for example via
EXECUTABLE_OUTPUT_PATH, add_test with the explicit COMMAND option will
substitute the correct path for the specified executable.
Merge pull request #4 from MikaelSmith/fix-test-paths

Enhance test paths
fixit no runnig tests in appveyor
Add option to skip tests

This can be used to skip tests on platforms where the stdlib
implementation appears to have a bug (i.e. Mac OS X and fstream
putback).
Merge pull request #6 from MikaelSmith/add-skip-test-option

Add option to skip tests