Add base and root bindings to pre test command

[Ni-g-3l]

Hey !

I found that on pre_test_command, "base" and "root" bindings were missing.

They are still available with "{this.base}" or "{this.root}", however I think it's important to keep consistency between available callback inside package.py

Feel free to comment or ask me question :)

[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 59.97%. Comparing base (dac6bcb) to head (57030ac). :warning: Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/package_test.py	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2032      +/-   ##
==========================================
- Coverage   60.11%   59.97%   -0.15%     
==========================================
  Files         163      163              
  Lines       20098    20120      +22     
  Branches     3494     3519      +25     
==========================================
- Hits        12082    12067      -15     
- Misses       7205     7232      +27     
- Partials      811      821      +10     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.