[newchem-cpp] mv grtest machinery into its own subdirectory

[mabruzzo]

This PR should be reviewed after #471 has been merged

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This has been on my todo list for quite a while.

This PR primarily makes 3 interrelated changes pertaining to the grtest machinery¹:

1. renames the grtest machinery its now called grtestutils
2. moves the grtest machinery out of tests/unit into test/grtestutils
3. lightly refactors the CMake logic (and add additional explanatory comments) so that instead of linking against each test file against either grtestutils or testdeps, we now link them all against testdeps

This PR also:

• addresses a TODO item from grtestutils/cmd.cpp (it reduces code duplication)
• explicitly links all test-code against the gmock library. While this doesn't have an immediate payoff, it will be a benefit for PR [newchem-cpp] cleanup test_status_reporting.cpp #472 and another PR. To be clear: THIS DOES NOT INTRODUCE ANY DEPENDENCIES²

Footnotes

1. Recall: the grtest machinery defines logic used to help implement unit tests. ↩

2. For added context, the gmock library is a component of googletest that is already being compiled as part of googletest. In other words, gmock was already an implicit dependency. By making it an explicit dependency, our test-code can access additional "matcher" functionality. (This functionality is most useful for testing strings and "containers") ↩