crimson/tests: fix test_remap_pin_concurrent

crimson/tests: fix test_remap_pin_concurrent

The test failed in the following sequence:

  1. 32 coroutines start in parallel, all calling try_get_pin(t, offset)
     with the same offset
  2. One coroutine succeeds in submit_transaction, which calls
     test_mappings.consume(delta) and modifies the shared test_mappings
  3. Other coroutines that haven't reached try_get_pin yet now hit the
     assertion because offset no longer exists in test_mappings

The fix is to add a guard before try_get_pin to check if test_mappings still
contains the offset. If it doesn't, we can safely exit early since that
means another coroutine already succeeded and modified the shared state.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>