Add PrepareForTestWithBuildFlag

Add a helper function that creates a test fixture preparer that
sets a build flag, and use it everywhere that was setting build flags
manually.

Test: all soong tests
Flag: EXEMPT refactor
Change-Id: I68d50d68787a30d091f0827e8caa51f5c5a762ef
diff --git a/android/config_test.go b/android/config_test.go
index ca7c7f8..7732168 100644
--- a/android/config_test.go
+++ b/android/config_test.go
@@ -150,12 +150,7 @@
 
 	for _, tc := range testCases {
 		fixture := GroupFixturePreparers(
-			FixtureModifyProductVariables(func(vars FixtureProductVariables) {
-				if vars.BuildFlags == nil {
-					vars.BuildFlags = make(map[string]string)
-				}
-				vars.BuildFlags["RELEASE_ACONFIG_EXTRA_RELEASE_CONFIGS"] = tc.flag
-			}),
+			PrepareForTestWithBuildFlag("RELEASE_ACONFIG_EXTRA_RELEASE_CONFIGS", tc.flag),
 		)
 		actual := fixture.RunTest(t).Config.ReleaseAconfigExtraReleaseConfigs()
 		AssertArrayString(t, tc.name, tc.expected, actual)