test(context): Introduce an explicit value merging test

Introduces a test which will merge variables defined at every possible
layer together and ensure that the loaded context configuration is as
expected.

The test data provides an actual resource set template that can be
tested locally from a kontemplate source checkout:

    kontemplate template context/testdata/merging/context.yaml --var cliVar=cliVar

context/testdata/merging/context.yaml

@@ -0,0 +1,15 @@
+# This context file is intended to test the merge hierarchy of
+# variables defined at different levels.
+---
+context: merging.in.kontemplate.works
+global:
+  globalVar: globalVar
+  includeVar: should be overridden (global)
+  cliVar: should be overridden (global)
+import:
+  - import-vars.yaml
+include:
+  - name: resource
+    values:
+      includeVar: includeVar
+      cliVar: should be overridden (include)

context/testdata/merging/import-vars.yaml

@@ -0,0 +1,4 @@
+importVar: importVar
+globalVar: should be overridden (import)
+includeVar: should be overridden (import)
+cliVar: should be overridden (import)

context/testdata/merging/resource/default.yaml

@@ -0,0 +1,5 @@
+defaultVar: defaultVar
+importVar: should be overridden (default)
+globalVar: should be overridden (default)
+includeVar: should be overridden (default)
+cliVar: should be overridden (default)

context/testdata/merging/resource/output.yaml

@@ -0,0 +1,5 @@
+defaultVar: {{ .defaultVar }}
+importVar: {{ .importVar }}
+globalVar: {{ .globalVar }}
+includeVar: {{ .includeVar }}
+cliVar: {{ .cliVar }}