scalars: speed up test and mark as “medium” (#2327)

Summary:
Addresses performance issues within Google: <http://b/134690650>. This
speeds up the test by 20–25%.

Test Plan:
That the test still passes suffices.

wchargin-branch: scalars-test-speed

tensorboard/plugins/scalar/BUILD

@@ -29,7 +29,7 @@ py_library(
 
 py_test(
     name = "scalars_plugin_test",
-    size = "small",
+    size = "medium",
     srcs = ["scalars_plugin_test.py"],
     main = "scalars_plugin_test.py",
     srcs_version = "PY2AND3",

tensorboard/plugins/scalar/scalars_plugin_test.py

@@ -40,7 +40,7 @@
 @test_util.run_v1_only('Requires contrib for db writer or uses op.Placeholder')
 class ScalarsPluginTest(tf.test.TestCase):
 
-  _STEPS = 99
+  _STEPS = 9
 
   _LEGACY_SCALAR_TAG = 'ancient-values'
   _SCALAR_TAG = 'simple-values'
@@ -101,9 +101,10 @@ def generate_run_to_db(self, experiment_name, run_name):
 
     with tf.compat.v1.Session() as sess:
       sess.run(tf.contrib.summary.summary_writer_initializer_op())
+      summaries = tf.contrib.summary.all_summary_ops()
       for step in xrange(self._STEPS):
         feed_dict = {global_step: step}
-        sess.run(tf.contrib.summary.all_summary_ops(), feed_dict=feed_dict)
+        sess.run(summaries, feed_dict=feed_dict)
       sess.run(flush_op)
 
   def testRoutesProvided(self):