Cleanup fuzzer methods to be reused

There were common methods between the fuzzers and
its gtets. Move that code to a common place where
both can use them to deduplicate code.

Since we are refactoring:
 * Create the art::fuzzer namespace
 * Remove the GC from the gtest as it doesn't
   make sense over there

Bug: 430216770
Test: Build and run verification/classes fuzzers
Test: m test-art-host-gtest-art_runtime_tests64
Flag: EXEMPT testing only
Change-Id: I8b2df8b34db14020f0c069aed1b4708067ae2f63