Share buildDir for android/soong/android tests

There is no need to create a separate buildDir for each test
file, use TestMain to create a global one for the package.

Test: all soong tests
Change-Id: I435ee7aa88b7e0bb8ccc1ba79f82833a7accf3e9

android/neverallow_test.go

@@ -15,8 +15,6 @@
 package android
 
 import (
-	"io/ioutil"
-	"os"
 	"testing"
 )
 
@@ -151,12 +149,6 @@ var neverallowTests = []struct {
 }
 
 func TestNeverallow(t *testing.T) {
-	buildDir, err := ioutil.TempDir("", "soong_neverallow_test")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(buildDir)
-
 	config := TestConfig(buildDir, nil)
 
 	for _, test := range neverallowTests {