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GH-40702: [R] Avoid undocumented dbplyr internals in duckdb tests (#40710)

### Rationale for this change

Most R CI is failing because of a dbplyr update that updated the internals of the intermediate representation. This was also a problem the last time dbplyr was updated where I believe it was me that hacked the fix of just using the *new* internal structure.

### What changes are included in this PR?

This fix eliminates the use of the dbplyr internals for that test, instead scraping the SQL query for the table name.

### Are these changes tested?

Covered by existing tests

### Are there any user-facing changes?

No
* GitHub Issue: #40702

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Nic Crane <thisisnic@gmail.com>