Introduce @env_spec for declarative test environments

[LukeMathWalker]

A building block which, later on, can be used by the scheduling pipeline to maximise env reuse across tests in the same suite.

You can see it in action in RediSearch/RediSearch#9706

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 59.45946% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.13%. Comparing base (02cfbb9) to head (888a59b).
⚠️ Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
RLTest/__main__.py	0.00%	18 Missing ⚠️
RLTest/loader.py	0.00%	11 Missing ⚠️
RLTest/env_spec.py	97.72%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   32.46%   35.13%   +2.67%     
==========================================
  Files          17       18       +1     
  Lines        2597     2755     +158     
==========================================
+ Hits          843      968     +125     
- Misses       1754     1787      +33     

Flag	Coverage Δ
unittests	35.13% <59.45%> (+2.67%)	⬆️

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