Provide cachegrind mode. (#1071)
Provide cachegrind mode. (#1071)

Motivation:

Cachegrind is nice to get more stable performance numbers, but it is dog
slow. We want to run our tests for shorter periods of time when using
cachegrind.

Modifications:

- When -DCACHEGRIND is provided, reduce the test iteration count to 1.

Results:

Cheaper test performance.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift