[Testing] Fix RH updates

**Moved flash asserts from tests to page.**

Currently, the test fails because there is a flash assert for "_Customer Information successfully saved_" both in page and test - this PR takes care of that and introduces a few more flash msg checks.

**Downstream only.**

### To test:
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**py.test -k register** (runtime: 5min - RHSM with http proxy fails in 5.2.4.x due to BZ1102724, but 5.2.3.x is full PASS)
**py.test -k update_appliances --long-running** (runtime: hours - but @mfalesni could stop by and check the results - whatchasay?)