patch 8.0.0478: tests use assert_true(0) and assert_false(1) to report errors

Problem:    Tests use assert_true(0) and assert_false(1) to report errors.
Solution:   Use assert_report().
diff --git a/src/testdir/test_gui.vim b/src/testdir/test_gui.vim
index 3e06eab..f044f3a 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -505,7 +505,7 @@
       " Case 2: guifontset is invalid
       try
         set guifontset=-*-notexist-*
-        call assert_false(1, "'set guifontset=-*-notexist-*' should have failed")
+        call assert_report("'set guifontset=-*-notexist-*' should have failed")
       catch
         call assert_exception('E598')
       endtry