fix e2e-test make target

[nicknovitski]

• explicitly wait for api token
• only use posix shell conditionals

What this PR does / why we need it:

The e2e-test make target is run with sh, not bash, which means that the operator =~ is not available, so the guard clause doesn't actually work.

output from ci:

if  [ "kubernetes-admin@ingress-nginx-dev" != "minikube" ] && \
		[ "kubernetes-admin@ingress-nginx-dev" =~ .*kind* ] && \
		[ "kubernetes-admin@ingress-nginx-dev" != "dind" ] && \
		[ "kubernetes-admin@ingress-nginx-dev" != "docker-for-desktop" ]; then \
		echo "kubectl context is "kubernetes-admin@ingress-nginx-dev", but must be one of [minikube, *kind*, dind, docker-for-deskop]"; \
		exit 1; \
	fi
/bin/sh: 2: [: kubernetes-admin@ingress-nginx-dev: unexpected operator

Another ci problem that's incredibly annoying is that the target has a chance to fail on the kubectl run command because the service account doesn't have a valid api token yet.

output from ci:

kubectl run --rm -i --tty \
		--attach \
		--restart=Never \
		--generator=run-pod/v1 \
		--env="E2E_NODES=10" \
		--env="FOCUS=.*" \
		--env="SLOW_E2E_THRESHOLD=50" \
		--overrides='{ "apiVersion": "v1", "spec":{"serviceAccountName": "ingress-nginx-e2e"}}' \
		e2e --image=nginx-ingress-controller:e2e
Error from server (ServerTimeout): No API token found for service account "ingress-nginx-e2e", retry after the token is automatically created and added to the service account
make: *** [e2e-test] Error 1
make: Leaving directory `/home/travis/build/kubernetes/ingress-nginx'
The command "test/e2e/run.sh" exited with 2.

Special notes for your reviewer: spun out from #4055

[aledbf]

/lgtm
/approve

[aledbf]

@nicknovitski thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, nicknovitski

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