TestConcurrentExcise is flaky

[RaduBerinde]

The test that was recently added is flaky:

go test -tags invariants -run ConcurrentExcise -exec 'stress -p 4'


        testdata/concurrent_excise:287:
         
        output didn't match expected:
        @@ -1,8 +1,3 @@
        -L0.0:
        -  000008:[a#12,SET-a#12,SET]
        -  000010:[d#13,SET-e#14,SET]
         L6:
        -  000005:[a#10,SET-a#10,SET]
        -  000012:[b#16,SET-bb#16,SET]
        -  000006:[e#11,SET-e#11,SET]
        +  000016:[a#0,SET-e#0,SET]