Make it easier to maintain the imported JS testsuites

tests/auto/qscriptjstestsuite and tests/auto/qscriptv8testsuite do pretty much the same thing (run an imported JS test suite as a Qt autotest), but duplicate a lot of code. It's also difficult to update the expected test results, since they're embedded in a C++ file and you must fiddle with defines and recompile the test to generate new expected results.

The tests should be refactored to share infrastructure, and should use external configuration files for defining skipped tests and expected failures. This will make it easier to adapt the tests when the QtScript back-end changes (e.g. to a more recent version of JavaScriptCore, or to V8). It will also make it easier to import other JS test suites (such as sputnik and es5conform) into the Qt autotests.