Threads may execute their function any time after they are spawned, so
coverage could be non-deterministic.

Fix this,

* for the script check worker threads by disabling them while fuzzing.
* for the scheduler thread by waiting for it to fully start and run the
  service queue.

When building with

 BUILD_FOR_FUZZING=OFF
 BUILD_FUZZ_BINARY=ON
 CMAKE_BUILD_TYPE=Debug

allow the fuzz binary to execute given test cases (without actual
fuzzing) to make it easier to reproduce fuzz test failures in a more
normal debug build.

In Debug builds, deterministic fuzz behaviour is controlled via a runtime
variable, which is normally false, but set to true automatically in the
fuzz binary, unless the FUZZ_NONDETERMINISM environment variable is set.

Co-Authored-By: Ryan Ofsky <ryan@ofsky.org>