Support test fixtures in etc package

Restructures the etc package test setup code to create FixturePreparer
instances for setting up a test fixture.

Bug: 181070625
Test: m nothing
Change-Id: I6f269c9cb3f4ab2756beddd13a719f9b222f7156
diff --git a/etc/prebuilt_etc.go b/etc/prebuilt_etc.go
index b07ad91..6291325 100644
--- a/etc/prebuilt_etc.go
+++ b/etc/prebuilt_etc.go
@@ -54,6 +54,8 @@
 	ctx.RegisterModuleType("prebuilt_dsp", PrebuiltDSPFactory)
 }
 
+var PrepareForTestWithPrebuiltEtc = android.FixtureRegisterWithContext(RegisterPrebuiltEtcBuildComponents)
+
 type prebuiltEtcProperties struct {
 	// Source file of this prebuilt. Can reference a genrule type module with the ":module" syntax.
 	Src *string `android:"path,arch_variant"`