e2e: tighten the chainsaw-transient test in the e2e sandbox image build #4887

The test that the e2e sandbox image build runs against its patched chainsaw has three loose ends. They were left out of #4867, which was already approved when they came up.

• The chainsaw subprocess has no deadline. A chainsaw that hangs on startup or on an API call blocks CombinedOutput, and the image build waits for the outer go test timeout. Running it through exec.CommandContext with a deadline of the assert timeout plus a minute, and a WaitDelay, fixes it. I checked that change on a Linux builder: the patched build passes, a build without the patch fails both tests, and a stub that hangs and leaves a child holding the output pipe fails after 75s with the deadline message.
• The skip guard in the Dockerfile greps for --- SKIP. A run where no test executes at all still reports ok, so it passes the guard.
• The vendored 0001-fix-assert-polling-error.patch has no -whitespace entry in .gitattributes, so git diff --check reports the whitespace inside the upstream patch. The cilium-operator and cluster-api-provider-kubevirt patches already have one.

Review threads: #4867 (comment), #4867 (comment), #4867 (comment)