ci: reduce coverage test failures

[yoomee1313]

• make coverage test sequential and double the time limit(30m -> 60m) for the test
• remove TestDiffLayer failure. referenced flaw in snapshot difflayer lookup ethereum/go-ethereum#27254

The sequential execution time of coverage is about 43m, so 60m time limit would be enough.
In the future, if it exceeds 60m, we can still double up. Or we can fix some tests which are allocated ports. (console tests, randao fork tests, etc)

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[blukat29]

Thanks!