spanner: Imports "testing"

[aaronbee]

spanner v1.29.0 imports the standard library "testing" package: https://github.com/googleapis/google-cloud-go/blob/spanner/v1.29.0/spanner/stats.go#L22

This adds an unwanted dependency to programs that import the spanner module. Imports of "testing" should be limited to _test.go files or test packages that don't get imported by production packages.

Expected behavior

$ go list -deps cloud.google.com/go/spanner | grep '^testing$'

should have an empty result

Actual behavior

$ go list -deps cloud.google.com/go/spanner | grep '^testing$'
testing

Additional context

Added in this PR: #5199

[codyoss]

Thanks @aaronbee! I have opened a PR to fix this.