Add --test-jar arg to test_runner to explicitly specify its path.

build/android/pylib/instrumentation/instrumentation_test_instance.py

Index: build/android/pylib/instrumentation/instrumentation_test_instance.py
diff --git a/build/android/pylib/instrumentation/instrumentation_test_instance.py b/build/android/pylib/instrumentation/instrumentation_test_instance.py
index f8d37e76c0223bc5b3f001268120ccd5cbca42e7..c2c1f4cb69f771e5b4016a627352391b4c30f7dc 100644
--- a/build/android/pylib/instrumentation/instrumentation_test_instance.py
+++ b/build/android/pylib/instrumentation/instrumentation_test_instance.py
@@ -487,16 +487,14 @@ class InstrumentationTestInstance(test_instance.TestInstance):
       assert self._suite.endswith('_incremental')
       self._suite = self._suite[:-len('_incremental')]
 
-    self._test_jar = os.path.join(
-        constants.GetOutDirectory(), constants.SDK_BUILD_TEST_JAVALIB_DIR,
-        '%s.jar' % self._suite)
+    self._test_jar = args.test_jar
     self._test_support_apk = apk_helper.ToHelper(os.path.join(
         constants.GetOutDirectory(), constants.SDK_BUILD_TEST_JAVALIB_DIR,
         '%sSupport.apk' % self._suite))
 
     if not os.path.exists(self._test_apk.path):
       error_func('Unable to find test APK: %s' % self._test_apk.path)
-    if not os.path.exists(self._test_jar):
+    if not self._test_jar or not os.path.exists(self._test_jar):
       error_func('Unable to find test JAR: %s' % self._test_jar)
 
     self._test_package = self._test_apk.GetPackageName()