Remove duplicate sdk.TestHelper

As part of the work on the new fixture mechanism some of the TestHelper
functionality was moved into the android/fixture.go package. This moves
the rest and removes the now duplicated TestHelper from the sdk
package.

Also removed some unnecessary & operators.

Bug: 181070625
Test: m nothing
Change-Id: Ia09a5d05e4fab3a4e28cf44b2d947a33541e3925
diff --git a/sdk/sdk_test.go b/sdk/sdk_test.go
index c4dc41b..8f885a1 100644
--- a/sdk/sdk_test.go
+++ b/sdk/sdk_test.go
@@ -390,7 +390,7 @@
 
 	extractor := newCommonValueExtractor(common)
 
-	h := TestHelper{t}
+	h := android.TestHelper{t}
 
 	err := extractor.extractCommonProperties(common, structs)
 	h.AssertDeepEquals("unexpected error", nil, err)
@@ -465,7 +465,7 @@
 
 	extractor := newCommonValueExtractor(common)
 
-	h := TestHelper{t}
+	h := android.TestHelper{t}
 
 	err := extractor.extractCommonProperties(common, structs)
 	h.AssertErrorMessageEquals("unexpected error", `field "S_Common" is not tagged as "arch_variant" but has arch specific properties: