Use shared envTest for unit tests

Speed up unit tests by using a shared envTest. This requires each
test to use its own namespace to avoid clobbering objects for
other tests. Tests previously took around 8 seconds each, and now
the initial test takes 2 seconds with follow up tests taking less
than a second each.

Also update existing tests that use a fixed namespace to use a
generated namespace.

Share gold file template function with yaml files.

Remove the testClusterMode, and instead rely on MainTest to do
the appropriate test setup and rootArgs flag setup. Move the
rootArg flag setup out of NewTestEnvKubeManager to avoid
side effects.

A follow up change can be to push the individual setups
from NewTestEnvKubeManager() into their respective TestMain since
the harness share little code.

Signed-off-by: Allen Porter <allen@thebends.org>

cmd/flux/testdata/trace/helmrelease.yaml

@@ -2,21 +2,21 @@
 apiVersion: v1
 kind: Namespace
 metadata:
-  name: flux-system
+  name: {{ .fluxns }}
 ---
 apiVersion: v1
 kind: Namespace
 metadata:
-  name: podinfo
+  name: {{ .ns }}
 ---
 apiVersion: helm.toolkit.fluxcd.io/v2beta1
 kind: HelmRelease
 metadata:
   labels:
     kustomize.toolkit.fluxcd.io/name: infrastructure
-    kustomize.toolkit.fluxcd.io/namespace: flux-system
+    kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
   name: podinfo
-  namespace: podinfo
+  namespace: {{ .ns }}
 spec:
   chart:
     spec:
@@ -24,7 +24,7 @@ spec:
       sourceRef:
         kind: HelmRepository
         name: podinfo
-        namespace: flux-system
+        namespace: {{ .fluxns }}
   interval: 5m
 status:
   conditions:
@@ -33,7 +33,7 @@ status:
     reason: ReconciliationSucceeded
     status: "True"
     type: Ready
-  helmChart: flux-system/podinfo-podinfo
+  helmChart: {{ .fluxns }}/podinfo-podinfo
   lastAppliedRevision: 6.0.0
   lastAttemptedRevision: 6.0.0
   lastAttemptedValuesChecksum: c31db75d05b7515eba2eef47bd71038c74b2e531
@@ -42,7 +42,7 @@ apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
 kind: Kustomization
 metadata:
   name: infrastructure
-  namespace: flux-system
+  namespace: {{ .fluxns }}
 spec:
   path: ./infrastructure
   sourceRef:
@@ -65,9 +65,9 @@ kind: GitRepository
 metadata:
   labels:
     kustomize.toolkit.fluxcd.io/name: flux-system
-    kustomize.toolkit.fluxcd.io/namespace: flux-system
+    kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
   name: flux-system
-  namespace: flux-system
+  namespace: {{ .fluxns }}
 spec:
   gitImplementation: go-git
   ref: