Support test fixtures in rust package

Replaces the rust specific rustTestCtx mechanism with the general test
fixtures mechanism as converting it to use preparers was not possible.

Also, removes usages of the buildDir variable and removes it as it is
no longer needed.

Bug: 181070625
Test: m nothing
Change-Id: I0176a7b6fb2d390ae23693f1e198da5124b4be63

rust/compiler_test.go

@@ -153,7 +153,7 @@ func TestLints(t *testing.T) {
 	for _, tc := range lintTests {
 		t.Run("path="+tc.modulePath, func(t *testing.T) {
 
-			config := android.TestArchConfig(buildDir, nil, bp, fs)
+			config := android.TestArchConfig(t.TempDir(), nil, bp, fs)
 			ctx := CreateTestContext(config)
 			ctx.Register()
 			_, errs := ctx.ParseFileList(".", []string{tc.modulePath + "Android.bp"})