Fix preflight test error message

[waynr]

What this PR does / why we need it:

It fixes a confusing error message in the preflight checks. It's not strictly
needed, but it is nice as a user when error messages make sense.

Which issue(s) this PR fixes

There is currently no issue for this problem.

Special notes for your reviewer:

Release note:

Fix dns preflight test error message

[johnSchnake]

💯 That is definitely confusing.

Bonus points if you're interested in expanding it to explain to the user what labels it was looking for/where. Although the message would now be more clear, I can imagine they'd immediately know what Sonobuoy considers a "dns pod" especially if they are experimenting with their own deployment or dns solution.

[johnSchnake]

CI will fail with this:

--- FAIL: TestDNSCheck (0.00s)
    --- FAIL: TestDNSCheck/Requires_at_least_one_pod (0.00s)
        preflight_test.go:168: Expected error to be "no dns pod tests found" but got "no dns pods found with the labels [foo] in namespace kube-system"
FAIL

I think everything else should be fine. After you push the unit test fix and CI is green I'll merge.

Thanks for the contribution!

[johnSchnake]

Sorry for the miscue; that second commit should be squashed into the first so that it is signed as well, otherwise the sign off check fails.

[johnSchnake]

💯 Thanks for the quick iterations on those. Merging since I validated the unit test signal locally.

[codecov-io]

Codecov Report

Merging #791 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
+ Coverage   42.59%   42.64%   +0.04%     
==========================================
  Files          71       71              
  Lines        4193     4193              
==========================================
+ Hits         1786     1788       +2     
+ Misses       2300     2298       -2     
  Partials      107      107

Impacted Files	Coverage Δ
pkg/client/preflight.go	57.35% <100%> (ø)	⬆️
cmd/sonobuoy/app/status.go	59.34% <0%> (+2.19%)	⬆️

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[waynr]

@johnSchnake thanks for the quick reviews!