Reactivate some tests and functionality that had accidentally been disabled due to incorrect checks

[multiplemonomials]

Summary of changes

• Some tests (crash-reporting and the mbed stats tests) would never be enabled, due to both incorrect TEST_SKIP issues and the fact that needed mbed_app.json options for these tests are disabled by default in Mbed. I added a new JSON file used for greentea that sets those options, and fixed the TEST_SKIP conditions
• Due to the earlier COMPONENT_FLASHIAP removal, the PlatformStorage API (which, to be honest, I had never seen before, but apparently is used as an intermediary between the storage layer and upper layers of Mbed) was erroneously thinking that the flash IAP block device wasn't enabled and wasn't using it when it should have.
• Crash reporting was not working on STM32H7 because it appears that the crash data was not getting written back to main memory before a reset. I fixed this by adding a cache clean operation.

Impact of changes

Migration actions required

Documentation

Updated the page that says how to run greentea tests locally.

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Verified locally that the newly added tests pass on STM32H7!

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