Add desugar library dependency to main test target.

This was otherwise not built in a --no-r8lib test run.

Change-Id: I657d4c51937a63321fdad36dc8d0d2167b82fa69
diff --git a/build.gradle b/build.gradle
index fba205d..9800595 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1822,6 +1822,7 @@
         systemProperty 'test_git_HEAD_sha1', project.property('HEAD_sha1')
     }
 
+    dependsOn buildLibraryDesugarConversions
     dependsOn getJarsFromSupportLibs
     // R8.jar is required for running bootstrap tests.
     dependsOn R8