Ignore user-defined runtime files in buffer test and rtfiles test

When initializing runtime files (syntax files etc) in tests, initialize
built-in runtime files only, to ensure that the tests are not affected
by whatever is in ~/.config/micro/ on the test machine.

micro_test.go already ensures that, by using its own temporary directory
as an (empty) config directory. So we only need to fix buffer_test.go
and rtfiles_test.go. In those tests, don't repeat the same dance with
a temporary directory, instead just ignore the config directory.

internal/buffer/buffer_test.go

@@ -20,7 +20,7 @@ type operation struct {
 
 func init() {
 	ulua.L = lua.NewState()
-	config.InitRuntimeFiles(true)
+	config.InitRuntimeFiles(false)
 	config.InitGlobalSettings()
 	config.GlobalSettings["backup"] = false
 	config.GlobalSettings["fastdirty"] = true

internal/config/rtfiles_test.go

@@ -7,7 +7,7 @@ import (
 )
 
 func init() {
-	InitRuntimeFiles(true)
+	InitRuntimeFiles(false)
 }
 
 func TestAddFile(t *testing.T) {