Use find_package to also include fprettify.tests

[awvwgk]

Running unit tests outside of the main repository fails since fprettify.tests is not installed.

Output from conda-build when trying to test fprettify installation:

## Package Plan ##

  environment location: /Users/runner/mambaforge/conda-bld/fprettify_1637846317519/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac

    fprettify:       0.3.7-pyhd8ed1ab_0        local      
    libffi:          3.4.2-h0d85af4_5          conda-forge
    libzlib:         1.2.11-h9173be1_1013      conda-forge
    ncurses:         6.2-h2e338ed_4            conda-forge
    openssl:         3.0.0-h0d85af4_2          conda-forge
    python:          3.10.0-h38b4d05_3_cpython conda-forge
    readline:        8.1-h05e3726_0            conda-forge
    sqlite:          3.36.0-h23a322b_2         conda-forge
    tk:              8.6.11-h5dbffcc_1         conda-forge
    tzdata:          2021e-he74cb21_0          conda-forge
    xz:              5.2.5-haf1e3a3_1          conda-forge
    zlib:            1.2.11-h9173be1_1013      conda-forge

Preparing transaction: ...working... done
Verifying transaction: ...working... done
Executing transaction: ...working... done
export PREFIX=/Users/runner/mambaforge/conda-bld/fprettify_1637846317519/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac
export SRC_DIR=/Users/runner/mambaforge/conda-bld/fprettify_1637846317519/test_tmp
import: 'fprettify'
import: 'fprettify'
+ fprettify --version
/Users/runner/mambaforge/conda-bld/fprettify_1637846317519/_test_env_placehold
_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeho
ld_placehold_placehold_placehold_placehold_placehold_placehold_placehold_place
hold_placehold_plac/bin/fprettify 0.3.7
+ python run_tests.py
Traceback (most recent call last):
  File "/Users/runner/mambaforge/conda-bld/fprettify_1637846317519/test_tmp/run_tests.py", line 22, in <module>
    from fprettify.tests import FPrettifyTestCase, FAILED_FILE, RESULT_FILE
ModuleNotFoundError: No module named 'fprettify.tests'
Tests failed for fprettify-0.3.7-pyhd8ed1ab_0.tar.bz2 - moving package to /Users/runner/mambaforge/conda-bld/broken