Ensure test table contains multiple active files

[findinpath]

Description

Ensure that the table created in the test does indeed have multiple files by partitioning the table.
Assuming that an unpartitioned table will have multiple files after being created with a CTAS statement can lead to flakiness in the tests.

This change is related to test infrastructure and doesn't change the existing functionality of the production code.

Related issues, pull requests, and links

Fixes #13107

[alexjo2144]

There's already one test with a partitioned table. Can we instead do a CTAS + INSERT to force multiple files?

[findepi]

I assume UNION (should be UNION ALL, really) was used to ensure multiple data files.

should we drop the second UNION branch now?

[findinpath]

Migrated the tests to use a CREATE TABLE statement and INSERT instead of CTAS as before.

[findepi]

i must admit i don't know what testMultiFileTable() is for.
Maybe we can remove it, or add a javadoc explaining its value
cc @alexjo2144

[alexjo2144]

I may have known at one point, but I'm also unsure if it has any value

[findepi]

if we don't know why we have a test, we don't know what's a good change and what isn't for the test prep or assertions.
let's drop it.

[findinpath]

I dropped the useless test in a separate PR.
testMultiFileTableWithNaNValue has been reworked to ensure that the table tested contains multiple active files.