UTs added for mount secret case, updated for config map as well

[bharathsreekanth]

Description

Unit tests updated for mount secret/credential. The test framework is a bit different as to enable more accurate reporting of the coverage. A single TestMain running both the secret and config map case reported coverage for only one of them (due to how go's coverage tool is).
A main test function now serializes how the tests are run that have a dependency of requiring a mock proxy server.

GitHub Issues

List the GitHub issues impacted by this PR:

| GitHub Issue # |
(dell/csm#1614)

Checklist:

• Have you run format,vet & lint checks against your submission?
• Have you made sure that the code compiles?
• Did you run the unit & integration tests successfully?
• Have you maintained at least 90% code coverage?
• Have you commented your code, particularly in hard-to-understand areas
• Have you done corresponding changes to the documentation
• Did you run tests in a real Kubernetes cluster?
• Backward compatibility is not broken

How Has This Been Tested?

Unit tests run for both secret and config map case.

PASS
coverage: 69.8% of statements
ok github.com/dell/csi-powermax/csireverseproxy/v2 60.492s coverage: 69.8% of statements