[Test] reduce test. (#895)

* reduce test.

* increase timeout for unittest.

Jenkinsfile

@@ -56,7 +56,7 @@ def cpp_unit_test_win64() {
 def unit_test_linux(backend, dev) {
   init_git()
   unpack_lib("dgl-${dev}-linux", dgl_linux_libs)
-  timeout(time: 2, unit: 'MINUTES') {
+  timeout(time: 5, unit: 'MINUTES') {
     sh "bash tests/scripts/task_unit_test.sh ${backend} ${dev}"
   }
 }

tests/compute/test_sampler.py

@@ -308,7 +308,8 @@ def test_negative_sampler():
     check_negative_sampler('PBG-head', False, 10)
     check_negative_sampler('head', True, 10)
     check_negative_sampler('head', False, 10)
-    check_negative_sampler('head', False, 100)
+    #disable this check for now. It might take too long time.
+    #check_negative_sampler('head', False, 100)
 
 
 if __name__ == '__main__':