Update run_tests.sh

to use the return code of ctest on osx and linux.

On windows using $? to get the return value is supposedly boken at least in minGW shells,
But it should work on unix systems.

tests/run_tests.sh

@@ -14,3 +14,5 @@ fi
 
 $CTEST_BINARY -D ExperimentalTest --no-compress-output
 cp Testing/`head -n 1 Testing/TAG`/Test.xml CTestResults.xml
+
+exit $?
\ No newline at end of file