Fix memory leak from ASAN checks

[wu-hui]

Newer version of ASAN started to support checking memory leaks.

• MaxValue() was leaking some heap memory because the special field value is allocated on heap and no one frees it. It's a negligible leak, but I fixed so we have clean ASAN by allocating the entire kMaxValue in static memory.
• Some tests in local_serializer_tests.cc are leaking memory, they happen to be tests we intend to remove, so I simply removed them. There are some production code we need to remove as well (local_store->DecodeMutationBatch), but I'll leave that for later.

[google-oss-bot]

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[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  Overall coverage changed from 87.60% (0bdc148) to 87.54% (1cd1bca) by -0.06%.

  Filename	Base (0bdc148)	Merge (1cd1bca)	Diff
  local_serializer.cc	90.25%	86.07%	-4.18%
  value_util.cc	95.65%	95.59%	-0.07%
  write_stream.cc	90.14%	94.37%	+4.23%

Test Logs

• Base (0bdc148): https://github.com/firebase/firebase-ios-sdk/actions/runs/2266678012
• Merge (1cd1bca): https://github.com/firebase/firebase-ios-sdk/actions/runs/2271892372

1. https://storage.googleapis.com/firebase-sdk-metric-reports/qEaMibfS8u.html