[BUG]: undefined Boost::headers with Boost::boost found

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### What version (or hash if on master) of pybind11 are you using?

4d0fced

### Problem description

in the tests/CMakeLists.txt the target Boost::headers is empty if Boost::boost found. I believe the problem is at https://github.com/pybind/pybind11/blob/4d0fcedc266df1f254da832d86698dfef15bfdf6/tests/CMakeLists.txt#L333 where the TARGET should be Boost::headers, instead of Boost::boost.

### Reproducible example code

```text
I use a custom install of Boost 1.71.0 with CMake 3.12.1 on a system (CentOS7) with Boost < 1.51. The custom Boost is found during CMake but the system library is included during make.
```


### Is this a regression? Put the last known working version here if it is.

Not a regression

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