Fix several bugs in cache_test

[rtjohnso]

test_wait_inflight did not match how it was used from test_do_read_batch.

Also, cache_assert_ungot was attempting to implement an impossible check and the test was attempting to use that check.

clockcache_assert_ungot was trying to check only that the current thread did not hold a read lock on the specificied page. But, since CC_RC_WIDTH is only 4, multiple threads may share the same read counter and hence, even if one thread holds no locks, the read counter for a page may be non-zero. So I changed clockcache_assert_ungot to check that no thread has a read lock on the page, which matches its description.

I then modified cache_test to use the corrected cache_assert_ungot.

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[rtjohnso]

Fixes #306 .