fix(cloud-databases): Refactor tests Part 1

[lornakelly]

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Ticket: https://github.ibm.com/compose/App/issues/1412

Refactoring acceptance tests is time consuming as running the tests takes a long time so refactoring bit by bit

• Removed common test functions into new file and out of PG file
• concatenate PG PITR test

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccIBMDatabaseInstancePostgresPITR'
=== RUN   TestAccIBMDatabaseInstancePostgresPITR
=== PAUSE TestAccIBMDatabaseInstancePostgresPITR
=== CONT  TestAccIBMDatabaseInstancePostgresPITR
--- PASS: TestAccIBMDatabaseInstancePostgresPITR (1266.04s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/database        1270.265s
...

[omaraibrahim]

LGTM