Fix flag checks to allow disabling unsecure tests

[kivaisan]

Description (required)

Fixed checking of configuration for unsecure tests to allow easier disabling of it. Now unsecure tests can be disabled by just removing MBED_CONF_APP_WIFI_UNSECURE_SSID configuration from json (similar to secure tests).

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

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)

This is just a test set up configuration change. In CI both secure and unsecure are tested but after this fix, it is easier to run just secure tests.

[X] 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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Reviewers (optional)

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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