Fix cache test to not rely on current context

[tejal29]

Skaffold Build Cache tests github.com/GoogleContainerTools/skaffold/pkg/skaffold/build/cache fails on master at or above b0f4b03 if kubeconfig context is set to minikube.

Expected behavior

go test ./... should pass irrespective of what the current context is set to in user's development environment

Actual behavior

tejaldesai@tejaldesai~/go/src/github.com/GoogleContainerTools/skaffold: (master)$ kubectl config use-context minikube
Switched to context "minikube".
tejaldesai@tejaldesai~/go/src/github.com/GoogleContainerTools/skaffold: (master)$ kubectl config current-context
minikube

tejaldesai@tejaldesai~/go/src/github.com/GoogleContainerTools/skaffold: (master)$ go clean --testcache
tejaldesai@tejaldesai~/go/src/github.com/GoogleContainerTools/skaffold: (master)$ go test -v github.com/GoogleContainerTools/skaffold/pkg/skaffold/build/cache -run ^Test_NewCache
=== RUN   Test_NewCache
=== RUN   Test_NewCache/get_a_valid_cache_from_file
time="2019-03-12T15:05:51-07:00" level=info msg="no config entry found for kube-context minikube"
=== RUN   Test_NewCache/needs_push
time="2019-03-12T15:05:51-07:00" level=info msg="no config entry found for kube-context minikube"
=== RUN   Test_NewCache/valid_cache_file_exists,_but_useCache_is_false
=== RUN   Test_NewCache/corrupted_cache_file
time="2019-03-12T15:05:51-07:00" level=warning msg="Error retrieving artifact cache, not using skaffold cache: yaml: unmarshal errors:\n  line 1: cannot unmarshal !!str `corrupt...` into cache.ArtifactCache"
--- FAIL: Test_NewCache (0.00s)
    --- FAIL: Test_NewCache/get_a_valid_cache_from_file (0.00s)
        cache_test.go:148: Expected result different from actual result. Expected: &{map[hash:{digest id}] 0xc00028c0a0 <nil> [{0 0 image map[]  [] [] 0 0 0}] /tmp/827158023 true false false}, Actual: &{map[hash:{digest id}] 0xc00028c140 <nil> [{0 0 image map[]  [] [] 0 0 0}] /tmp/827158023 true false true}
    --- FAIL: Test_NewCache/needs_push (0.00s)
        cache_test.go:148: Expected result different from actual result. Expected: &{map[hash:{digest id}] 0xc0000caf50 <nil> [] /tmp/649495994 true true false}, Actual: &{map[hash:{digest id}] 0xc0000cafa0 <nil> [] /tmp/649495994 true true true}
    --- PASS: Test_NewCache/valid_cache_file_exists,_but_useCache_is_false (0.00s)
    --- PASS: Test_NewCache/corrupted_cache_file (0.00s)
FAIL
FAIL	github.com/GoogleContainerTools/skaffold/pkg/skaffold/build/cache	0.117s
tejaldesai@tejaldesai~/go/src/github.com/GoogleContainerTools/skaffold: (master)$ 

Information

• Skaffold version: skaffold @ b0f4b03
• Operating system: ...
• Contents of skaffold.yaml:

<paste your skaffold.yaml here>

Steps to reproduce the behavior

1. set kubecontext to minikube.
2. run go clean --testcache
3. run go test -v github.com/GoogleContainerTools/skaffold/pkg/skaffold/build/cache -run ^Test_NewCache