For spanner GA. Investigate whether these are already tested, and add tests as appropriate. Link either PR # or line reference of existing test.

• Read single key
  https://github.com/GoogleCloudPlatform/google-cloud-python/blob/6c602823e7101b40dfe5bfc614da633beb8accfb/spanner/tests/system/test_system.py#L840-L850
• Read single key w/ index (Spanner: Read single key with index #4403)
• Range read w/ single key (Spanner: multiple ranges single key #4573)
• Range read w/ partial key (Spanner: System test partial range #4631)
• Range read over index w/ open/open range (Spanner: Range read with keys and index #4411)
• Range read over index w/ open/closed range (Spanner: Range read with keys and index #4411)
• Range read over index w/ closed/open range (Spanner: Range read with keys and index #4411)
• Range read over index w/ closed/closed range (Spanner: Range read with keys and index #4411)
• Bind string to null (Add spanner system tests: #4371)
• Bind timestamp (Add spanner system tests: #4371)
• Bind date https://github.com/GoogleCloudPlatform/google-cloud-python/blob/f69d704841bd30c08af4a522397b3fe95107bdfd/spanner/tests/system/test_system.py#L673-L679
• Bind string array (Add spanner system tests: #4371)
• Bind empty string array (Add spanner system tests: #4371)
• Query returning array of -Inf, +Inf, Nan (Add spanner system tests: #4371)
• Write / read back null string (Add spanner system tests: #4372)
• Write / read back empty array of string (Add spanner system tests: #4372)
• Write / read back null array of string (Add spanner system tests: #4372)
• Write / read back array of string with mixed null / non-null values (Add spanner system tests: #4372)
• Writing incorrect type of value should fail (Spanner: invalid type test #4569 )

/cc @lukesneeringer