Dump kubectl stderr when it fails in TestHelloWorldFromShell

[adrcunha]

Error doesn't contain this useful debugging information.

Fixes #1530.

[google-prow-robot]

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[adrcunha]

Example of test failure:

$ go test -v -tags=e2e -count=1 ./test/e2e -run ^TestHelloWorldFromShell$
info	logging/config.go:90	Successfully created the logger.	{"knative.dev/jsonconfig": "{\n\t  \"level\": \"info\",\n\t  \"encoding\": \"console\",\n\t  \"outputPaths\": [\"stdout\"],\n\t  \"errorOutputPaths\": [\"stderr\"],\n\t  \"encoderConfig\": {\n\t    \"messageKey\": \"message\",\n\t\t\t\"levelKey\": \"level\",\n\t\t\t\"nameKey\": \"logger\",\n\t\t\t\"callerKey\": \"caller\",\n\t\t\t\"messageKey\": \"msg\",\n      \"stacktraceKey\": \"stacktrace\",\n      \"lineEnding\": \"\",\n      \"levelEncoder\": \"\",\n      \"timeEncoder\": \"\",\n      \"durationEncoder\": \"\",\n      \"callerEncoder\": \"\"\n\t  }\n\t}"}
info	logging/config.go:91	Logging level set to info
=== RUN   TestHelloWorldFromShell
info	TestHelloWorldFromShell	e2e/helloworld_shell_test.go:68	Creating manifest
info	TestHelloWorldFromShell	e2e/helloworld_shell_test.go:92	Manifest file is '/tmp/helloworld226820215'
info	TestHelloWorldFromShell	e2e/helloworld_shell_test.go:93	Deploying using kubectl
info	TestHelloWorldFromShell	e2e/helloworld_shell_test.go:58	Sleeping for 20 seconds after Route deletion to avoid hitting issue in #1376
--- FAIL: TestHelloWorldFromShell (23.05s)
	helloworld_shell_test.go:97: Error running kubectl: unable to recognize "/tmp/helloworld226820215": no matches for serving.knative.dev/, Kind=Configuration
		unable to recognize "/tmp/helloworld226820215": no matches for serving.knative.dev/, Kind=Route
FAIL
FAIL	github.com/knative/serving/test/e2e	23.179s

[adrcunha]

/hold

Wait until after we cut first release.

[jessiezcc]

/lgtm

[srinivashegde86]

/lgtm

[adrcunha]

/hold cancel