chore: simplify test tempdir

[crenshaw-dev]

Follow-up to simplify this test: #8508 (comment)

I also added a test to make sure we hit this error:

	if !strings.HasPrefix(certPath, dataPath) {
		return nil, fmt.Errorf("could not get certificate for host %s", serverName)
	}

In other words, check for a directory traversal.

[codecov]

Codecov Report

Merging #9152 (29bb39e) into master (d2f5060) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9152      +/-   ##
==========================================
- Coverage   45.52%   45.48%   -0.05%     
==========================================
  Files         219      219              
  Lines       25882    25882              
==========================================
- Hits        11783    11772      -11     
- Misses      12458    12472      +14     
+ Partials     1641     1638       -3     

Impacted Files	Coverage Δ
applicationset/services/scm_provider/github.go	63.52% <0.00%> (-17.65%)	⬇️
applicationset/services/scm_provider/utils.go	88.50% <0.00%> (+4.59%)	⬆️

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[jannfis]

LGTM, thanks!