Merge function and end2end testcases into a single set

[andy-maier]

Today, the function testcases and the end2end testcases have separate test code, but perform tests at the same level, whereby the function tests build a mocked HMC in the test code, and the end2nd tests work against a real HMC, but both test against nearly the complete stack of zhmc ansible module code.

This duplicates the effort for writing test code.

Eliminate the duplication by having one set of test functions that is run against mocked HMCs and against real HMCs. The zhmcclient.testutils support in zhmcclient 1.2.0 (that was already used in PR #510) provides the basis for that.

Staging:

• Stage 1: POC with the zhmc_user module (since that has fairly complete end2end test code). The resulting test code should not miss anything that was done in the function or end2end test code.
• Stage 2: Convert all other modules.

[andy-maier]

PR #535 enables running the end2end tests against a mocked environment, with all existing end2end testcases (zhmc_partition, zhmc_user). This PR does not yet merge the function and end2end testcases, but I think it still shows how this would work and thus demonstrates the POC from stage 1 defined above.

In stage 2, the function testcases should be merged into the end2end testcases using this concept.

[andy-maier]

PR #544 merges the function tests for list modules into the end2end tests.