fix(bigtable): Retry correct mutations

[bhshkh]

b/387315971

TestMutateRows_Retry_TransientErrors fails with below errors:

=== RUN   TestMutateRows_Retry_TransientErrors
    mutaterows_test.go:287: diff found (-want +got):
          (*bigtablepb.MutateRowsRequest)(Inverse(protocmp.Transform, protocmp.Message{
          	"@type": s"google.bigtable.v2.MutateRowsRequest",
          	"entries": []protocmp.Message{
          		{
          			"@type":     s"google.bigtable.v2.MutateRowsRequest.Entry",
          			"mutations": []protocmp.Message{{"@type": s"google.bigtable.v2.Mutation", "set_cell": protocmp.Message{"@type": s"google.bigtable.v2.Mutation.SetCell", "column_qualifier": []uint8("col"), "family_name": string("f"), "timestamp_micros": int64(1000), ...}}},
          			"row_key": []uint8{
          				... // 2 identical elements
          				0x77,
          				0x2d,
        - 				0x31,
        + 				0x32,
          			},
          		},
          		{
          			"@type":     s"google.bigtable.v2.MutateRowsRequest.Entry",
          			"mutations": []protocmp.Message{{"@type": s"google.bigtable.v2.Mutation", "set_cell": protocmp.Message{"@type": s"google.bigtable.v2.Mutation.SetCell", "column_qualifier": []uint8("col"), "family_name": string("f"), "timestamp_micros": int64(1000), ...}}},
          			"row_key": []uint8{
          				... // 2 identical elements
          				0x77,
          				0x2d,
        - 				0x32,
        + 				0x33,
          			},
          		},
          	},
          	"table_name": string("projects/project/instances/instance/tables/table"),
          }))
    mutaterows_test.go:292: diff found (-want +got):
          (*bigtablepb.MutateRowsRequest)(Inverse(protocmp.Transform, protocmp.Message{
          	"@type": s"google.bigtable.v2.MutateRowsRequest",
          	"entries": []protocmp.Message{
          		{
          			"@type":     s"google.bigtable.v2.MutateRowsRequest.Entry",
          			"mutations": []protocmp.Message{{"@type": s"google.bigtable.v2.Mutation", "set_cell": protocmp.Message{"@type": s"google.bigtable.v2.Mutation.SetCell", "column_qualifier": []uint8("col"), "family_name": string("f"), "timestamp_micros": int64(1000), ...}}},
          			"row_key": []uint8{
          				... // 2 identical elements
          				0x77,
          				0x2d,
        - 				0x31,
        + 				0x33,
          			},
          		},
          	},
          	"table_name": string("projects/project/instances/instance/tables/table"),
          }))
--- FAIL: TestMutateRows_Retry_TransientErrors (0.15s)

These are requests and responses sent by the mock server setup by the test
Tests send requests for 4 row mutations (row-0, row-1, row-2, row-3). The mock server returns 'Unavailable' error for row-1 and row-2.
Expected: The library should retry mutations for row-1 and row-2
Actual: The library retries mutations for row-2 and row-3
Cause: The library assumes that responses are returned in the same order of the requests. This is not true.
The server may return the response in any order. The Index field should be used to determine the request for which the response was sent. In this test, for req [row-0, row-1, row-2, row-3], the response sent by the server is [{Index: 0, Err:nil}, {Index: 3, Err:nil}, {Index: 1, Err: Unavailable}, {Index: 2, Err: Unavailable}]. The library incorrectly assumes one on one mapping of error array to req array i.e. it assumes that unavailable response was received for row-2 and row-3 and retries them.
Fix: This PR fixes the test by using Index field to determine which rows need to be retried

After fix:

=== RUN   TestMutateRows_Retry_TransientErrors
--- PASS: TestMutateRows_Retry_TransientErrors (0.12s)
PASS
ok  	github.com/googleapis/cloud-bigtable-clients-test/tests	0.137s