Refactor StorageQuotaFull test cases in test_quota

There were two test cases in test_quota that shared the same name, but
did almost the same thing. After a bit of digging, it looked like most
of the test cases did the almost same thing, so I refactored the common
bits and fixed the test case name duplication.

Partial-Bug: #1259036
Change-Id: I828eea570d358d98520c7201454c03ba174a35ad
Merge "Refactor StorageQuotaFull test cases in test_quota"