Reactivate kvstore and general filesystem Greentea tests

[multiplemonomials]

Summary of changes

This PR reactivates a few tests under the kvstore/ folder. These are some of the last tests that haven't been set up in CMake yet! (full list in #2). Note that some of these tests are too large to fit on a few targets, so I had to add a new "MIN_FLASH_BYTES" option for greentea_add_test() to filter them out.

I also noticed and fixed two other small issues while making this PR:

• My MAX32670 PR from a month ago missed one of the newly added files for OpenOCD support, so re-adding it
• Test executables in some directories weren't being built because of parent directories being set as EXCLUDE_FROM_ALL in CMake. This meant that our greentea compile check CI job wasn't actually checking that every test compiled!

Impact of changes

• KVStore tests are now active in CMake (at least, for targets which have DEVICE_TRNG).
  • Someday I'd like to enable these tests for non-TRNG targets as well but it requires adding some new code to set up a "root of trust" in each test case that uses TRNG features.
• Greentea tests in excluded folders will now be built by CMake

Migration actions required

Documentation

None

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] 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)
[] Tests / results supplied as part of this PR

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