Fix more tests when ANDROID_BUILD_TOP is set

The bpfix code (also used by androidmk) reads the ANDROID_BUILD_TOP
environment variable, causing the tests to fail when it is set.  Clear
it in the bpfix and partner androidmk tests the same way the main
androidmk tests do.

Test: go test ./...
Change-Id: Icc61f2c3ebdbecab34c1d18399c6846db207c6dd
diff --git a/partner/androidmk/androidmk_test.go b/partner/androidmk/androidmk_test.go
index 6bae836..3ace750 100644
--- a/partner/androidmk/androidmk_test.go
+++ b/partner/androidmk/androidmk_test.go
@@ -54,6 +54,9 @@
 }
 
 func TestEndToEnd(t *testing.T) {
+	// Skip checking Android.mk path with cleaning "ANDROID_BUILD_TOP"
+	t.Setenv("ANDROID_BUILD_TOP", "")
+
 	for i, test := range testCases {
 		expected, err := bpfix.Reformat(test.expected)
 		if err != nil {