test: Skip desugar with DESUGAR=false when running tests

Allow one to skip desugar when running the tests, for example:

   $> ANDROID_COMPILE_WITH_JACK=false DESUGAR=false DX=<your-dx-tool> art/test.py

Why this might be useful: If building with javac and a dexer tool that
already processes invoke-dynamic lambdas, then desugar is redundant.

Bug: 36902714
Change-Id: I39aadca4985e2aafe4b4b24ff3dc9d7a364bbad7
diff --git a/test/run-test b/test/run-test
index ba1f992..1b6df16 100755
--- a/test/run-test
+++ b/test/run-test
@@ -91,6 +91,11 @@
 
 export JACK="$JACK -g -cp $JACK_CLASSPATH"
 
+# Allow changing DESUGAR script to something else, or to disable it with DESUGAR=false.
+if [ -z "$DESUGAR"]; then
+  export DESUGAR="$ANDROID_BUILD_TOP/art/tools/desugar.sh"
+fi
+
 # Zipalign is not on the PATH in some configs, auto-detect it.
 if [ -z "$ZIPALIGN" ]; then
   if which zipalign >/dev/null; then