fix(spanner): ensure defers run at the right time in tests (#9759)

* fix(spanner): ensure defers run at the right time in tests

Defers are executed at the end of the func, not at the end of the loop.
This adds func closures to ensure that they execute within the single
test.

* chore(spanner): convert closures to subtests

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Co-authored-by: rahul2393 <[email protected]>

Author: egonelbre