Pass Config to NewTestContext instead of ctx.Register

Prepare for using Config when adding singletons by passing
Config to NewTestContext and NewContext instead of to ctx.Register.
This will enable a followup change to store SingletonMakeVarsProviders
registered on the Context in the Config, which is necessary to run
multiple tests in parallel without data races.

Test: all soong tests
Change-Id: Id229629a4e42ff4487d317241673837726c075fc
diff --git a/rust/compiler_test.go b/rust/compiler_test.go
index a25523c..a6dba9e 100644
--- a/rust/compiler_test.go
+++ b/rust/compiler_test.go
@@ -152,8 +152,8 @@
 		t.Run("path="+tc.modulePath, func(t *testing.T) {
 
 			config := android.TestArchConfig(buildDir, nil, bp, fs)
-			ctx := CreateTestContext()
-			ctx.Register(config)
+			ctx := CreateTestContext(config)
+			ctx.Register()
 			_, errs := ctx.ParseFileList(".", []string{tc.modulePath + "Android.bp"})
 			android.FailIfErrored(t, errs)
 			_, errs = ctx.PrepareBuildActions(config)