ASSERT fcache.c:3241 TEST(FRAG_FOLLOWS_FREE_ENTRY, subseq->flags)

PR #4430 hit this assert when it changed the usable size of 64-bit cache units from 56K (b/c of 2 guard pages subtracted from 64K) to 64K. It hit it on these 4 tests, all of which run the same multi-threaded app:

[00:33:50]      198 - code_api|tool.drcachesim.threads (Failed)
[00:33:50]      199 - code_api|tool.drcachesim.threads-with-config-file (Failed)
[00:33:50]      200 - code_api|tool.drcachesim.coherence (Failed)
[00:33:50]      201 - code_api|tool.drcachesim.TLB-threads (Failed)

[00:31:22] 198: <Application C:\projects\dynamorio\build\build_debug-internal-64\suite\tests\bin\client.annotation-concurrency.exe (4360).  Internal Error: DynamoRIO debug check failure: ..\..\core\fcache.c:3241 TEST(FRAG_FOLLOWS_FREE_ENTRY, subseq->flags)
[00:31:22] 198: (Error occurred @14222 frags)

I couldn't reproduce locally: I ran TLB-threads 20x and all 20 passed.

This seems to be non-deterministic: re-running on Appveyor caused all 4 asserts to disappear! A separate tree focusing on this in PR #4432 hit no assert running 1 test, then 3 out of 4 when running 4 tests.

Shrinking all the cache unit sizes back to 56K causes the assert to disappear, so far as I can tell.

This is a little disturbing, that there's a bug in the fcache logic after all this time. Without a local repro it's going to be hard to figure out though.