Greentea: Fix slice-calculation from kvstore test phase 2

[adbridge]

Summary of changes

This PR pulls in a single commit (Greentea: Fix slice-calculation from kvstore test phase 2) from Master which is required for 5.15 but the full PR from which it was originally added, is a feature one so cannot be taken.

Impact of changes

Migration actions required

Documentation

None

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)

---

Test results

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

---

Reviewers

---

[adbridge]

@SeppoTakalo @0xc0170 I have pulled this across directly to 5.15 as it is outside the capabilities of the release script and we need it before generating RC 2. Please review. It cannot be labelled with a release version as that would cause the script to try and pull it in again.

[adbridge]

Required on mbed-os-5.15 by #12050

[adbridge]

@0xc0170 This should have gone in before RC2. It looks like we may need to produce an RC3 after this and then #12050 is brought across.

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 5 of 5 test jobs passed
Build number : 1
Build artifacts

[adbridge]

Re-running ci to ensure no conflicts.

[mbed-ci]

Test run: SUCCESS

Summary: 5 of 5 test jobs passed
Build number : 2
Build artifacts

[adbridge]

This is not a blocker and may only be reconsidered if we make a 5.15.1 and feel that the fix is required. Closing for now as the fix is on master