Remove string concatenation in spanner DB update

[modular-magician]

b/174602915

Updates with multiple statements were formatted incorrectly resulting in an API error. This PR removes the statement concatenation that causes this and also modifies the update test to capture this scenario.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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Release Note Template for Downstream PRs (will be copied)

spanner: fixed an issue in `google_spanner_database` where multi-statement updates were not formatted correctly

Derived from GoogleCloudPlatform/magic-modules#4294