Add test fixture support

Adds the test fixture support and converts a few tests to exercise the
code and show how it works.

Bug: 181070625
Test: m nothing
Change-Id: I0a2b40fff93b6041f9aa8c4ef0aba91da1bc8bf3
diff --git a/android/Android.bp b/android/Android.bp
index a8fa53a..8f6c9ad 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -34,6 +34,7 @@
         "deptag.go",
         "expand.go",
         "filegroup.go",
+        "fixture.go",
         "hooks.go",
         "image.go",
         "license.go",
@@ -87,6 +88,7 @@
         "depset_test.go",
         "deptag_test.go",
         "expand_test.go",
+        "fixture_test.go",
         "license_kind_test.go",
         "license_test.go",
         "licenses_test.go",