Skip Upgrade tests for PostgreSQL

Problem/Motivation

In #710858-61: Meta issue: fix the remaining PostgreSQL bugs it looks like we're almost there with getting PostgreSQL tests to pass in D7.

However, some of the Upgrade tests have been failing inconsistently.

Steps to reproduce

Run D7 tests with PostgreSQL.

Proposed resolution

At this stage in D7's lifecycle we could skip the upgrade tests in PostgreSQL.

To be clear, there are Upgrade tests that check upgrades from D6 to D7. It's these we're proposing we skip with PostgreSQL.

There are also Update tests that check D7's update hooks. We will not skip these in PostgreSQL.

Remaining tasks

Add code to skip the problematic Upgrade tests for PostgreSQL.

User interface changes

n/a

API changes

n/a

Data model changes

n/a

Release notes snippet

n/a ?