[Bug]: TESTONLY targets don't appear to get installed

### Describe the issue

I'm trying to build and install Abseil with CMake and use a `PUBLIC` target that's marked `TESTONLY`.

https://github.com/abseil/abseil-cpp/blob/c3b5022604551a045e383c68071d7be0a807839d/absl/log/CMakeLists.txt#L618-L620

After building and installing, I do not see a cmake target named `absl::scoped_mock_log`.

### Steps to reproduce the problem

I fetch build and install abseil in a temporary location as follows:

```console
$ git clone https://github.com/abseil/abseil-cpp
$ cd abseil-cpp
$ cmake -S. -Bcmake-out -DCMAKE_BUILD_TYPE=Release -GNinja -DCMAKE_CXX_STANDARD=14 -DABSL_BUILD_TESTING=ON -DABSL_USE_GOOGLETEST_HEAD=ON
$ cmake --build cmake-out
$ cmake --install cmake-out --prefix /tmp/absl
```

Then I look for a `absl::scoped_mock_log` CMake target

```console
$ find /tmp/absl/ -name '*.cmake'
/tmp/absl//lib/cmake/absl/abslTargets.cmake
/tmp/absl//lib/cmake/absl/abslConfig.cmake
/tmp/absl//lib/cmake/absl/abslTargets-release.cmake
$ grep absl::scoped_mock /tmp/absl//lib/cmake/absl/abslTargets.cmake
$
```
You can grep that file for a target like, `absl::optional` to see that it _does_ exist.

### What version of Abseil are you using?

$ git show-ref HEAD
d6ea4df62db59dc1ebc1fd3b62146700640ef400 refs/remotes/origin/HEAD

### What operating system and version are you using?

macOS 13.2.1

### What compiler and version are you using?

$ cc --version
Apple clang version 14.0.0 (clang-1400.0.29.202)
Target: arm64-apple-darwin22.3.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

### What build system are you using?

$ cmake --version
cmake version 3.25.1

CMake suite maintained and supported by Kitware (kitware.com/cmake).

### Additional context

From looking at the this file

https://github.com/abseil/abseil-cpp/blob/d6ea4df62db59dc1ebc1fd3b62146700640ef400/CMake/AbseilHelpers.cmake#L49-L50

I think I'm required to add `ABSL_BUILD_TESTING=ON` and `BUILD_TESTING=ON` (the default) in order to get `TESTONLY` targets to work.

I think this is probably a misfeature. I think every target that is considered a public API point that users may want to use should be built and installed by default, even without explicitly enabling testing.