(PDOC-9) Switch to metadata.json for test module

Prior to this commit, the test module used in testing was using
a Modulefile rather than metadata.json. Switch to metadata.json
instead since Modulefile is deprecated and was causing a deprecation
warning during acceptance testing.

spec/unit/puppet/examples/test/Modulefile

@@ -1,10 +0,0 @@
-name          'username-test'
-version       '0.1.0'
-source        ''
-author        'username'
-license       'Apache 2.0'
-summary       ''
-description   ''
-project_page  ''
-
-dependency 'puppetlabs/stdlib'

spec/unit/puppet/examples/test/metadata.json

@@ -0,0 +1,6 @@
+{
+  "name": "username-test",
+  "version": "0.0.1",
+  "author": "username",
+  "license": "Apache 2.0"
+}