test/robustness: rename makefile.mk to Makefile

[ivanvc]

This idea came from a robustness meeting where @serathius explained how the robustness tests Makefile was structured. Renaming the file to Makefile allows users to execute make commands from the tests/robustness directory. However, to also be able to import it from the top-level Makefile, we need to ensure that we are not using relative directories. This is simple to achieve by introducing variables to the repository's top-level, and executing the top-level make/Makefile from the robustness by using make's --directory argument.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

/cc @serathius

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.86%. Comparing base (82a3e6f) to head (be3eee2).
Report is 18 commits behind head on main.

Additional details and impacted files

see 19 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19594      +/-   ##
==========================================
+ Coverage   68.81%   68.86%   +0.05%     
==========================================
  Files         421      421              
  Lines       35897    35897              
==========================================
+ Hits        24703    24722      +19     
+ Misses       9768     9753      -15     
+ Partials     1426     1422       -4     

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[ivanvc]

/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[serathius]

This is really awesome!