logictest: alter_table logictest is flaky

[adityamaru]

=== CONT  TestLogic/fakedist/alter_table
    logic.go:2498: 
         
        /go/src/github.com/cockroachdb/cockroach/pkg/sql/logictest/testdata/logic_test/alter_table:86: SELECT job_type, regexp_replace(description, 'JOB \d+', 'JOB ...'), user_name, fraction_completed::decimal(10,2)
        FROM crdb_internal.jobs
        WHERE job_type = 'SCHEMA CHANGE' OR job_type = 'SCHEMA CHANGE GC'
        ORDER BY created DESC
        LIMIT 3
        expected:
            SCHEMA CHANGE GC  GC for temporary index used during index backfill                           root  0.00
            SCHEMA CHANGE GC  GC for ROLLBACK of ALTER TABLE test.public.t ADD CONSTRAINT bar UNIQUE (c)  root  0.00
            SCHEMA CHANGE     ALTER TABLE test.public.t ADD CONSTRAINT bar UNIQUE (c)                     root  0.00
            
        but found (query options: "") :
            SCHEMA CHANGE GC  GC for temporary index used during index backfill                           root  1.00
            SCHEMA CHANGE GC  GC for ROLLBACK of ALTER TABLE test.public.t ADD CONSTRAINT bar UNIQUE (c)  root  0.00
            SCHEMA CHANGE     ALTER TABLE test.public.t ADD CONSTRAINT bar UNIQUE (c)                     root  0.00

Seen on https://teamcity.cockroachdb.com/viewLog.html?buildId=4434087&buildTypeId=Cockroach_UnitTests

[blathers-crl]

cc @cockroachdb/bulk-io

[adityamaru]

Haven't had a chance to verify that this actually flakes regularly, but thought I'd capture it since I saw it on my bors batch.