test: Fix intermittent issue in interface_rest.py

[maflcko]

Fixes:

 test  2024-02-22T16:15:37.465000Z TestFramework (ERROR): Assertion failed 
  Traceback (most recent call last):
  File "/ci_container_base/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/test_framework/test_framework.py", line 131, in main
   self.run_test()
  File "/ci_container_base/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/interface_rest.py", line 340, in run_test
   assert_equal(json_obj, mempool_info)
  File "/ci_container_base/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/test_framework/util.py", line 57, in assert_equal
   raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
 AssertionError: not({'loaded': True, 'size': 3, 'bytes': 312, 'usage': 3600, 'total_fee': Decimal('0.00093600'), 'maxmempool': 300000000, 'mempoolminfee': Decimal('0.00001000'), 'minrelaytxfee': Decimal('0.00001000'), 'incrementalrelayfee': Decimal('0.00001000'), 'unbroadcastcount': 1, 'fullrbf': False} == {'loaded': True, 'size': 3, 'bytes': 312, 'usage': 3600, 'total_fee': Decimal('0.00093600'), 'maxmempool': 300000000, 'mempoolminfee': Decimal('0.00001000'), 'minrelaytxfee': Decimal('0.00001000'), 'incrementalrelayfee': Decimal('0.00001000'), 'unbroadcastcount': 0, 'fullrbf': False})

https://cirrus-ci.com/task/4852944378527744?logs=ci#L4436

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Type	Reviewers
ACK	mzumsande, m3dwards

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[mzumsande]

ACK faeed91

The problem seems to be that the existing sync_all waits until the mempool sizes are identical, but the call to RemoveUnbroadcastTx() is executed right after sending the tx in ProcessGetData(), so nothing waits for that.
An alternative fix might be to switch the order in ProcessGetData() (call RemoveUnbroadcastTx() right before MakeAndPushMessage() instead of right after).

[m3dwards]

ACK faeed91

Tested on ARM Mac.

I personally haven't seen that flake yet but @mzumsande's explanation for why it can happen makes sense to me and this seems like a pragmatic solution to the test failing.