Test batching

Add support for --batch-tests argument to the configure script. When provided, all tests will be merged into a single executable, due to long linking times/binary size on certain platforms, including WASM.

When first such target is being processed, a new combined target for all of the future test sources is created under the name of 'test_batch'. CMake attempts to merge the parameters of each of the tests, and some basic checks are run for parameter differences that are impossible to reconcile.

Parameterless run lists all of the available test cases. First parameter is interpreted as the test to run.