Add apexFixtureFactory to apex package

Unlike the similar changes in other packages this change separates the
addition of the fixture factory and the conversion of the test...
methods to use them as there are a few tests that need converting to
use test fixtures first.

Bug: 181070625
Test: m nothing
Change-Id: Ic76523ba89fc1967631aeb682935935b5af116df
diff --git a/apex/Android.bp b/apex/Android.bp
index 8a2edeb..1890b89 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -25,6 +25,7 @@
         "deapexer.go",
         "key.go",
         "prebuilt.go",
+        "testing.go",
         "vndk.go",
     ],
     testSrcs: [