remove spanner::MakeTestRow() when deprecation period expires

[devbww]

As of 2022-06-01, google::cloud::spanner::MakeTestRow() has been deprecated in favor of the identically-functioning google::cloud::spanner_mocks::MakeRow().

This is part of an effort to move test-only code to test-only libraries.

The old interface will be removed from the code base on or shortly after 2023-06-01, so users will need to migrate any affected code before then. Deprecation attributes have been added to the old API to aid in its identification. See the diffs between the old and new calls in #9080 for a more concise picture of how to perform migrations.

When the deprecation period expires we should:

• move the implementation to spanner_mocks::MakeRow(), replacing the forwarding calls, and remove the disable_deprecation_warnings wrapping, and
• remove spanner::MakeTestRow() and its GOOGLE_CLOUD_CPP_SPANNER_MAKE_TEST_ROW_DEPRECATED() attribute macro.

[mpeddada1]

Fixed in #15850. This will be included with v3.x.y.