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

cc/cc_test.go

@@ -3194,12 +3194,7 @@ func TestVendorSdkVersion(t *testing.T) {
 
 	ctx = android.GroupFixturePreparers(
 		prepareForCcTest,
-		android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
-			if variables.BuildFlags == nil {
-				variables.BuildFlags = make(map[string]string)
-			}
-			variables.BuildFlags["RELEASE_BOARD_API_LEVEL_FROZEN"] = "true"
-		}),
+		android.PrepareForTestWithBuildFlag("RELEASE_BOARD_API_LEVEL_FROZEN", "true"),
 	).RunTestWithBp(t, bp)
 	testSdkVersionFlag("libfoo", "30")
 	testSdkVersionFlag("libbar", "29")