Tests: Make test_patcher not require a real patchelf in /usr/bin

[lengau]

Discovered this accidentally thanks to a ~/.bashrc that put /bin before /usr/bin in $PATH.

I'm not entirely sure this was unintentional, so unless someone else knows we should probably wait for @sergiusens.

[codecov-commenter]

Codecov Report

Merging #4003 (a917454) into main (31a3990) will increase coverage by 0.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4003      +/-   ##
==========================================
+ Coverage   90.99%   91.61%   +0.61%     
==========================================
  Files          56       77      +21     
  Lines        4154     5153     +999     
==========================================
+ Hits         3780     4721     +941     
- Misses        374      432      +58     

Impacted Files	Coverage Δ
snapcraft/providers.py	96.55% <0.00%> (ø)
snapcraft/store/client.py	95.43% <0.00%> (ø)
snapcraft/parts/yaml_utils.py	92.68% <0.00%> (ø)
snapcraft/commands/__init__.py	100.00% <0.00%> (ø)
snapcraft/ua_manager.py	96.38% <0.00%> (ø)
snapcraft/errors.py	91.17% <0.00%> (ø)
snapcraft/__main__.py	0.00% <0.00%> (ø)
snapcraft/meta/__init__.py	100.00% <0.00%> (ø)
snapcraft/plugins/v1/__init__.py	0.00% <0.00%> (ø)
snapcraft/__init__.py	72.72% <0.00%> (ø)
... and 14 more

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