test(spanner): run slow integration tests in nightly build (#5815)
test(spanner): run slow integration tests in nightly build (#5815)

Run the "slow" Spanner integration tests in "integration-nightly" to
increase coverage without impacting normal pre/post-merge testing.

Before:
//google/cloud/spanner/integration_tests:backup_integration_test    0.4s
//google/cloud/spanner/samples:samples                            137.8s

After:
//google/cloud/spanner/integration_tests:backup_integration_test  676.2s
//google/cloud/spanner/samples:samples                            766.8s
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp