Build testing

[henryiii]

The original implementation (and many implementations of BUILD_TESTING, I think) has an issue; if you set BUILD_TESTING in a parent project, it gets picked up unconditionally in a child project. This makes BUILD_TESTING be ignored if CLI11 is a child project; CLI11_TESTING will still control it if a parent project really wants a child project to build its tests. If you set BUILD_TESTING with CLI11 as the parent project, this will override CLI11_TESTING, as it does now.

Based on plans mentioned in #273, follows up on #183. This may be the last change for 1.8.

Also adds clang-format testing on Azure, along with GCC 4.7, 9, Clang 3.4, 8. You can now force libc++ if this is the master project.

Drops experimental optional auto-support, it can be fragile.

Main Project	CLI11_TESTING unset	CLI11_TESTING OFF	CLI11_TESTING ON
BUILD_TESTING default	✔️	❌	✔️
BUILD_TESTING OFF	❌	❌	✔️
BUILD_TESTING ON	✔️	❌	✔️

Sub Project	CLI11_TESTING unset	CLI11_TESTING OFF	CLI11_TESTING ON
BUILD_TESTING unset	❌	❌	✔️
BUILD_TESTING OFF	❌	❌	✔️
BUILD_TESTING ON	❌	❌	✔️

[codecov]

Codecov Report

Merging #277 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #277   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        2782   2782           
=====================================
  Hits         2782   2782

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0be526e...e4e5d6f. Read the comment docs.

[codecov]

Codecov Report

Merging #277 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #277   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        2782   2782           
=====================================
  Hits         2782   2782

Impacted Files	Coverage Δ
include/CLI/Timer.hpp	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0be526e...a3113e4. Read the comment docs.

[phlptp]

Will the other format check be dropped. Seems a little redundant to have both, or are they checking different things?

[henryiii]

I'll be slowly reducing our usage of non-Azure CI, so yes. I could probably go ahead and drop that in this PR.

[henryiii]

I'm actually still only checking "clang-tidy" on Travis. I'll need to move that in here in the future.