patch 8.1.1619: tests are not run with GUI on Travis

Problem:    Tests are not run with GUI on Travis.
Solution:   Add a testgui job. (Ozaki Kiichi, closes #4609)
diff --git a/.travis.yml b/.travis.yml
index 8e12240..b669a90 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -214,5 +214,12 @@
         - *linux-huge
         - *asan
       after_failure: *asan_symbolize
+    - <<: *linux
+      name: huge-testgui+coverage/gcc
+      compiler: gcc
+      env:
+        - *linux-huge
+        - TEST="-C src testgui"
+      after_success: *coverage
 
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