test: add method for setting plugin path for cmd

When running a PackerCommand for acceptance tests, we generally run the
test on a temporary plugin directory, populated by test plugins.

Setting that temporary directory means we need to set the environment
variable, which while it could be easier with a constant for the name
for example, isn't too straightforward.

Therefore for those tests we add a new function for PackerCommand so
that it automatically sets that envvar for the current command.

test/commands_test.go

@@ -33,6 +33,11 @@ func (ts *PackerTestSuite) PackerCommand() *packerCommand {
 	}
 }
 
+// UsePluginDir sets the plugin directory in the environment to `dir`
+func (pc *packerCommand) UsePluginDir(dir string) *packerCommand {
+	return pc.AddEnv("PACKER_PLUGIN_PATH", dir)
+}
+
 func (pc *packerCommand) SetArgs(args ...string) *packerCommand {
 	pc.args = args
 	return pc

test/loading_test.go

@@ -37,7 +37,7 @@ func (ts *PackerTestSuite) TestLoadingOrder() {
 			t.Run(tt.name, func(t *testing.T) {
 				ts.PackerCommand().
 					SetArgs(command, tt.templatePath).
-					AddEnv("PACKER_PLUGIN_PATH", pluginDir).
+					UsePluginDir(pluginDir).
 					Assert(t, MustSucceed{}, Grep{
 						streams: BothStreams,
 						expect:  tt.grepStr,