[TESTS][SQL] Setup testdata at the beginning for tests to run independently

## What changes were proposed in this pull request?

In CachedTableSuite, we are not setting up the test data at the beginning. Some tests fail while trying to run individually. When running the entire suite they run fine.

Here are some of the tests that fail -

- test("SELECT star from cached table")
- test("Self-join cached")

As part of this simplified a couple of tests by calling a support method to count the number of
InMemoryRelations.

## How was this patch tested?

Ran the failing tests individually.

Author: Dilip Biswal <dbiswal@us.ibm.com>

Closes #16688 from dilipbiswal/cachetablesuite_simple.