WiFi: Re-order Greentea tests with minor fixes

[AriParkkila]

Description (required)

WiFi test case changes:

• Run WIFI-GET-RSSI and WIFI-CONNECT-DISCONNECT-REPEAT tests in secure mode
• Remove WIFI-CONNECT-PARAMS-VALID-UNSECURE, it's duplicate to WIFI_CONNECT with remains moved to WIFI-CONNECT-SECURE

Summary of change (What the change is for and why)

Prefer testing in secure mode as it's much more common.

Documentation (Details of any document updates required)

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Pull request type (required)

[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 (required)

[] 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

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Reviewers (optional)

@kivaisan

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

@AriParkkila, thank you for your changes.
@kivaisan @ARMmbed/mbed-os-test @ARMmbed/mbed-os-ipcore @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

@AriParkkila Can you amend your commit msg to add there these:

Run WIFI-GET-RSSI and WIFI-CONNECT-DISCONNECT-REPEAT tests in secure mode
Remove WIFI-CONNECT-PARAMS-VALID-UNSECURE, it's duplicate to WIFI_CONNECT with remains moved to WIFI-CONNECT-SECURE

[AriParkkila]

@0xc0170 fixed commit msg.

[kimlep01]

I tested this PR overnight (9 times) with one of our Wifi boards and saw four times less test failures on average.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

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

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[0xc0170]

Test failed because of #11862, we will investigate

[adbridge]

CI restarted

[mbed-ci]

Test run: SUCCESS

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

[adbridge]

This is sitting on top of changes for #11409 which is currently targeting 6.0