Consolidate Au CMake targets

[chiphogg]

This breaks the 1:1 correspondence between target and test, so we split
up our helper accordingly into two.

We make au the single mega-target that holds everything that doesn't
depend on GTest. It turns out that there are two targets that do.
testing is for external users, and contains custom GMock matchers.
chrono_policy_validation is an internal-only library that makes it
easy to test that Au is like chrono where we want to be, and unlike
chrono otherwise. Each of these targets also depends on the au
mega-target.

Helps #257.

[chiphogg]

Note to the reviewer:

This PR uses "checkpoint commits" to make it easier to verify correctness. Currently, there are 4. Start at the first commit, and use n and p to go to next and previous. Here is what the commits do:

1. Bulk-update rules:
   1. Instead of header_only_library doing both the header and the test, we split it into two rules (by adding the new gtest_based_test), because tests and libraries aren't 1:1 anymore.
   2. We now have only three header_only_library instances: au (public, main); testing (public, optional); chrono_policy_validation (private).
   3. There are two aspects of this commit that make for easier diffing: (a) GTEST_SRCS is not renamed to SRCS yet, and (b) all of the HEADERS in au are on one single line, so that git isn't tempted to match them to their old rules.
   4. We get rid of GTEST_SRCS and GTEST_EXTRA_DEPS from the header-only libraries, because again, tests aren't 1:1 with library targets anymore.
   5. Every old header_only_library instance becomes a gtest_based_test
   6. _test gets appended to the name
   7. HEADERS goes away
   8. Old DEPS goes away
   9. New DEPS: delete any targets that went away, and add au (the new "mono-target") as a dep basically every time
2. Do the rest of the change.
   1. The HEADERS go one per line
   2. GTEST_SRCS gets renamed to just SRCS
3. Fix the breakage in the previous commit.
   1. Look. I ran all the tests before pushing. But CMake was so sloppy that it just reused the previously built executables! It's good that we're providing CMake support, but this just reinforces what a good thing it is that we're bazel-first.
4. Update docs!

[geoffviola]

Looks good. I can see that the dependency on gmock has moved.

The following test still looks good.

docker run --rm \
ubuntu@sha256:2e863c44b718727c860746568e1d54afd13b2fa71b160f5cd9058fc436217b30 \
/bin/bash -c \
'apt update && apt install -y clang cmake git && git clone -b chiphogg/consolidate-cmake-libs#257 https://github.com/aurora-opensource/au.git && cd au && cmake -S . -B cmake/build -DCMAKE_VERIFY_INTERFACE_HEADER_SETS=TRUE && cmake --build cmake/build -j $(proc) --target all all_verify_interface_header_sets && ctest -j $(nproc) --test-dir cmake/build'