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_popup.vim b/src/testdir/test_popup.vim
index 2823ed6..4789ca7 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -562,7 +562,7 @@
   %d
   try
     call feedkeys("o/*\<cr>\<cr>\<c-x>\<c-u>/\<esc>", 'tx')
-    call assert_false(1, 'completefunc not set, should have failed')
+    call assert_report('completefunc not set, should have failed')
   catch
     call assert_exception('E764:')
   endtry