refactor: unit test document hanlding

[hisImminence]

Which problem does the PR fix?

Trying to make the unit tests more generic and easier to read - I did the following refactorings:

1. Created 3 functions that could be used by all configmap unit tests actually --> they could be moved into a go package and reused in other configmap tests:
   • renderTemplate(): renders configmap template (used in all tests before)
   • verifyConfigMap(): compares expected vs. set (used in all tests before)
   • runTestCases(): runs all testCases (see point 2.)
2. Created a testCase structure instead of single test functions. This makes it very visible to see what values (in values.yaml) I set and which values (in this case envVars) I expect to set. The struct looks like:
   2.1. name of the testCase
   2.2. all values that we are setting in the values.yaml.
   2.3 all expected values (in this case envVars), which should be set after the configmap got rendered.

p.s. lets wait for https://github.com/camunda/camunda-platform-helm/pull/3141/files to be merged as it removes the local option.

What's in this PR?

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