test: fix flaky TestExactAccountChunk again

[cce]

Summary

I recently this test fail on an unrelated PR:

=== FAIL: ledger TestExactAccountChunk/future_SPstall300 (5.12s)
    catchpointfilewriter_test.go:910: 
                Error Trace:    /opt/cibuild/project/ledger/catchpointfilewriter_test.go:910
                                                        /opt/cibuild/project/ledger/catchpointfilewriter_test.go:872
                Error:          Not equal: 
                                expected: 0x320
                                actual  : 0x31f
                Test:           TestExactAccountChunk/future_SPstall300

It seems the issue is that in #6214, I added an assertion that the DoubleLedger's two ledgers had committed the same round (on the rnd returned by testCatchpointFlushRound) BEFORE the code added to address flakiness in #5993 and wait until the two ledgers were caught up and in sync. (It makes sense that this 300-round subtest is the one that failed, since there were more rounds waiting to be flushed.) This moves the assertion after the fix in #5993.

Test Plan

Existing test should pass, and hopefully not occasionally fail.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.66%. Comparing base (272ec02) to head (8423e83).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6278      +/-   ##
==========================================
+ Coverage   51.63%   51.66%   +0.03%     
==========================================
  Files         647      647              
  Lines       86907    86907              
==========================================
+ Hits        44876    44903      +27     
+ Misses      39161    39136      -25     
+ Partials     2870     2868       -2     

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