Make test_suites arch_variant

In most cases, we don't care about arm vs x86 when selecting whether a
module is in a test suite or not, but we do care about whether we need
to add the host or device module(s) to the suite. So enable the arch
mutator.

Test: Add android-specific test_suites property, look in out/soong/Android.mk
Change-Id: I1e258d6e97e18c8d20297568cd8ee8d4161d5023

cc/test.go

@@ -43,7 +43,7 @@ type TestBinaryProperties struct {
 
 	// list of compatibility suites (for example "cts", "vts") that the module should be
 	// installed into.
-	Test_suites []string
+	Test_suites []string `android:"arch_variant"`
 }
 
 func init() {