test: don't capitalize error string

[mainred]

What type of PR is this?

What this PR does / why we need it:

Error strings should not be capitalized (unless beginning with proper nouns or acronyms) or end with punctuation, since they are usually printed following other context

Which issue(s) this PR fixes:

Fixes #

Requirements:

• run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• includes documentation
• adds unit tests

How has this code been tested

Special notes for your reviewer

[codecov]

Codecov Report

Merging #127 (9ed6881) into main (63a876a) will increase coverage by 0.65%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
+ Coverage   78.12%   78.77%   +0.65%     
==========================================
  Files          20       20              
  Lines        2455     2455              
==========================================
+ Hits         1918     1934      +16     
+ Misses        422      409      -13     
+ Partials      115      112       -3     

Impacted Files	Coverage Δ
pkg/controllers/multiclusterservice/controller.go	84.24% <0.00%> (+1.09%)	⬆️
pkg/controllers/member/endpointslice/controller.go	80.64% <0.00%> (+2.82%)	⬆️
...rollers/member/internalserviceexport/controller.go	83.33% <0.00%> (+8.33%)	⬆️

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