fix(nightly): add timeout to two gc tests (#2801)

Compare a recent master paralleled run on buildkite with the sequential nightly on nightly instance:
parallel:
https://nightly.neartest.com/s3/master_68d77cbfc2939f28410102f88ec1d5fba6256806_20200605_181035
sequential:
https://nightly.neartest.com/run/master_68d77cbfc2939f28410102f88ec1d5fba6256806_20200605_110602

10 timeouts on parallel, 6 on sequential. 5 of them are overlapped, 1 test is timeout in sequential but didn't in parallel. 

5 test was only timeout in parallel:

- fixed because missing `--exact` wrongly run multiple tests:
```
expensive near-client catching_up tests::test_catchup_random_single_part_sync
expensive near-client cross_shard_tx tests::test_cross_shard_tx
expensive near-client catching_up tests::test_all_chunks_accepted_1000
```

- fix in this PR by adding 20% more time according to log, to be safe, adding 50%
```
expensive near-chain gc tests::test_gc_remove_fork_large
expensive near-chain gc tests::test_gc_random_large
```

Test Plan
---------
run nightly on buildkite, should have at most these sequential also timeout tests timeout:
```
expensive near-client catching_up tests::test_catchup_receipts_sync_hold
expensive near-client catching_up tests::test_chunk_grieving
expensive near-client cross_shard_tx tests::test_cross_shard_tx_drop_chunks
expensive near-chain gc tests::test_gc_star_large
pytest sanity/garbage_collection.py
```

Author: ailisp

nightly/nightly.txt

@@ -128,10 +128,10 @@ expensive nearcore test_cases_testnet_rpc test::test_delete_access_key_with_allo
 expensive nearcore test_cases_testnet_rpc test::test_access_key_smart_contract_testnet
 
 # GC tests
-expensive --timeout=600 near-chain gc tests::test_gc_remove_fork_large
+expensive --timeout=900 near-chain gc tests::test_gc_remove_fork_large
 expensive --timeout=1200 near-chain gc tests::test_gc_not_remove_fork_large
 expensive --timeout=1200 near-chain gc tests::test_gc_boundaries_large
-expensive --timeout=600 near-chain gc tests::test_gc_random_large
+expensive --timeout=900 near-chain gc tests::test_gc_random_large
 expensive --timeout=600 near-chain gc tests::test_gc_pine
 expensive --timeout=600 near-chain gc tests::test_gc_star_large