Prevent test262 repository update if not needed

[HalidOdat]

The previous implementation of clone_test262() was fetching the test262 repository on every run even if the commit did not change.

This also removes documentation section about test262 submodule.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	95,574	95,574	0
Passed	75,342	75,342	0
Ignored	19,482	19,482	0
Failed	750	750	0
Panics	0	0	0
Conformance	78.83%	78.83%	0.00%

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (3177540) 45.86% compared to head (31ba0cd) 45.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3386      +/-   ##
==========================================
- Coverage   45.86%   45.86%   -0.01%     
==========================================
  Files         477      477              
  Lines       48944    48950       +6     
==========================================
  Hits        22450    22450              
- Misses      26494    26500       +6     

Files	Coverage Δ
boa_tester/src/main.rs	0.00% <0.00%> (ø)

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