Unify WebAssembly test runners

Currently there are two separate test runners in qtbase:

• the batched test runner, used for running a subset of the Qt tests on CI
• qtwasmtestlib, used for running wasm-spesific tests.
   

We should unify these and support running the different types of auto-tests from a single test runner implementation.

Requirements:

• It should be possible to compile and run a single test (e.g. "ninja tst_qbytearray"). The  test is then picks up the build configuration from Qt (thread/no-thread, asyncify 0 - 1 - 2, etc)
• It should be possible to compile and run the CI test set in batched mode.
• emrun integration
• The test runner should provide a JavaScript API for running tests: listing test functions and running individual test functions (or all test functions). 
• The test runner should provide xml output.
• The test runner should provide a html front-end for running a single test.
• Use the new qtloader.js, instead of duplicating this functionality.
• Support UI tests (provide a container element / screen for the test)