Adds defaults for `--storageEngineCacheSizeGB` if not explicitly set.

A non-default is either configured in the target:
```
resmoke_suite_test(
resmoke_args = [
"--storageEngineCacheSizeGB=3",
],
...
```

or supplied via `--test_arg` on the bazel command:
`bazel test //my:suite -test_arg=-storageEngineCacheSizeGB=3`