conformance validation: Add new helper method in testutil package

[mayankshah1607]

Problem

The testutil package offers a set of utility functions and helpers for the integration tests. As a part of the Conformance Validation project (RFC), we plan on reusing these utility functions to avoid having to reinvent the wheel or make things redundant by re-writing the same helpers in a different package. How this can be done is by calling the testutil.NewTestHelper() method which returns a new instance of testutil.TestHelper, on which we call the utility functions.

However, the problem with this approach is that the testutil.NewTestHelper() requires you to pass a set of flags that are only useful to the integration tests. For example, the -integration-tests flag, which determines if the integration tests should be run, errors out if it is not set to true. The -linkerd flag, which determines the path of the linkerd2 binary, won't serve any purpose in the conformance tests, as we plan on using Sonbuoy, which pulls up a pod, that will always look at a fixed file path that points to the binary of a particular version which will be mentioned in the Sonobuoy plugin.
The -linkerd-namespace flag, which specifies the namespace of the control plane, is by default set to l5d-integration. Moreover, some of the other flags are irrelevant to conformance testing. The Conformance Validation project would most certainly benefit from reusing the testutil package without having to depend on these rules or flag options configured specifically for the integration tests.

How should this be solved?

We propose to have a new helper method testutil.NewConformanceTestHelper which is similar to testutil.NewTestHelper - it returns a new instance of testutil.TestHelper, but instead of depending on flag options, the values of the fields in testutil.TestHelper shall be passed as function arguments. This way, not only will we have full control over the configuration options from the conformance testing side while being able to reuse the utility functions from testutil package, but will also enhance UX by hiding away irrelevant flag options. This new method shall be used by the linkerd2-conformance repo. This will also ensure that the existing integration test workflow does not break.

cc @Pothulapati @grampelberg

[mayankshah1607]

I do not see any use case for this new helper method apart from conformance validation, hence the name NewConformanceTestHelper. But if we do plan on taking this approach, we could use general naming conventions like NewBaseHelper, or something alike.