spanner: more tests, part 1
spanner: more tests, part 1

Add tests to fulfill the Cloud Spanner client test plan.

These tests are translated from the first half of
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITReadTest.java.

Change-Id: Iaef1133cf5e1036e6d9ec074596f52e47b0d82f9
Reviewed-on: https://code-review.googlesource.com/12978
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Vikas Kedia <f6a34f50adc64ff8913356438469cce93f09537a@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go