scripts: Twister ConfigurationError Fix

Current implementation of ConfigurationError causes
an exception to be thrown.
It is because a PosixPath is being concatenated with an str with a '+'
operator, which is not permitted.
The path in question is now cast to an str.
Additional test has been added to test_twister.py to track regressions.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>
2 files changed