Prune test preparers in apex/boot_image_test.go

Cut down the preparers to only those needed by these tests.

Bug: 177892522
Test: m nothing
Change-Id: I811977b0331dd38c25623aa96e09e85ba5cf7c57
diff --git a/apex/apex_test.go b/apex/apex_test.go
index e0cefa1..bcbbb28 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -140,13 +140,13 @@
 			],
 		}
 	`),
+	prepareForTestWithMyapex,
 	android.FixtureMergeMockFs(android.MockFS{
-		"a.java":                                    nil,
-		"PrebuiltAppFoo.apk":                        nil,
-		"PrebuiltAppFooPriv.apk":                    nil,
-		"apex_manifest.json":                        nil,
-		"AndroidManifest.xml":                       nil,
-		"system/sepolicy/apex/myapex-file_contexts": nil,
+		"a.java":                 nil,
+		"PrebuiltAppFoo.apk":     nil,
+		"PrebuiltAppFooPriv.apk": nil,
+		"apex_manifest.json":     nil,
+		"AndroidManifest.xml":    nil,
 		"system/sepolicy/apex/myapex.updatable-file_contexts":         nil,
 		"system/sepolicy/apex/myapex2-file_contexts":                  nil,
 		"system/sepolicy/apex/otherapex-file_contexts":                nil,
@@ -204,6 +204,10 @@
 	}),
 )
 
+var prepareForTestWithMyapex = android.FixtureMergeMockFs(android.MockFS{
+	"system/sepolicy/apex/myapex-file_contexts": nil,
+})
+
 func setUp() {
 	var err error
 	buildDir, err = ioutil.TempDir("", "soong_apex_test")