test(spanner): add a MutationsTest for the Spanner types (#8751)
test(spanner): add a MutationsTest for the Spanner types (#8751)

Add a test case for Mutation marshalling of the Spanner wrapper
types, Bytes, Date, Json, Numeric, and Timestamp, which all end
up as string values.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp