Let tests override environment

Pass an environment to TestConfig that will be used for
all Config.Getenv calls.

Test: none
Change-Id: I683cd9c9e0db61c9bfd2adb27fca78f558f225c4

android/paths_test.go

@@ -209,7 +209,7 @@ func (m moduleInstallPathContextImpl) InstallInSanitizerDir() bool {
 }
 
 func TestPathForModuleInstall(t *testing.T) {
-	testConfig := TestConfig("")
+	testConfig := TestConfig("", nil)
 
 	hostTarget := Target{Os: Linux}
 	deviceTarget := Target{Os: Android}