patch 8.2.1184: some tests fail

Problem:    Some tests fail.
Solution:   Adjust tests for different assert_fails() behavior.  Remove unused
            variable.
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index c2d055c..0ec710a 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -208,12 +208,12 @@
 endfunc
 
 func Test_assert_fail_fails()
-  call assert_equal(1, assert_fails('xxx', {}))
-  call assert_match("Expected {} but got 'E731:", v:errors[0])
+  call assert_equal(1, assert_fails('xxx', 'E12345'))
+  call assert_match("Expected 'E12345' but got 'E492:", v:errors[0])
   call remove(v:errors, 0)
 
-  call assert_equal(1, assert_fails('xxx', {}, 'stupid'))
-  call assert_match("stupid: Expected {} but got 'E731:", v:errors[0])
+  call assert_equal(1, assert_fails('xxx', 'E9876', 'stupid'))
+  call assert_match("stupid: Expected 'E9876' but got 'E492:", v:errors[0])
   call remove(v:errors, 0)
 
   call assert_equal(1, assert_fails('echo', '', 'echo command'))