Update installation instruction regarding python tests

[azeey]

🦟 Bug fix

Fixes gazebosim/gazebo_test_cases#915

Summary

Python tests require make install.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #441 (3bd5f65) into gz-transport13 (d92aed2) will not change coverage.
Report is 1 commits behind head on gz-transport13.
The diff coverage is n/a.

❗ Current head 3bd5f65 differs from pull request most recent head 57d2df8. Consider uploading reports for the commit 57d2df8 to get more accurate results

@@               Coverage Diff               @@
##           gz-transport13     #441   +/-   ##
===============================================
  Coverage           87.76%   87.76%           
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  Files                  59       59           
  Lines                5696     5696           
===============================================
  Hits                 4999     4999           
  Misses                697      697