Allow adb shell to run as root in tests variant

Linaro is starting to use the TARGET_BUILD_VARIANT=tests, slowly
transitioning from the traditional TARGET_BUILD_VARIANT=eng builds.
Testing has found that LAVA (specifically the lava-android-test project)
expects adb shell to offer full root access. This is not the case in
'tests' builds where adb shell drops privileges and runs as the 'shell'
user.

Since it does not decrease security in any way (tests builds are marked
as insecure) would like to have adb root shell access in
TARGET_BUILD_VARIANT=tests as well.

Change-Id: I20cbe3f32385162f9dc2b6b71c47ad22b9a0924f
diff --git a/adb/Android.mk b/adb/Android.mk
index 3813c09..1b18a41 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -140,7 +140,7 @@
 endif
 LOCAL_CFLAGS += -D_XOPEN_SOURCE -D_GNU_SOURCE
 
-ifneq (,$(filter userdebug eng,$(TARGET_BUILD_VARIANT)))
+ifneq (,$(filter userdebug eng tests,$(TARGET_BUILD_VARIANT)))
 LOCAL_CFLAGS += -DALLOW_ADBD_ROOT=1
 endif