patch 9.0.1382: failing test for strptime() doesn't show returned value

Problem:    Failing test for strptime() doesn't show returned value.
Solution:   Use assert_equal() instead of assert_true().
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 1e2b378..b8500c5 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -4286,8 +4286,8 @@
   if exists_compiled('*strptime')
     v9.CheckDefAndScriptFailure(['strptime(10, "2021")'], ['E1013: Argument 1: type mismatch, expected string but got number', 'E1174: String required for argument 1'])
     v9.CheckDefAndScriptFailure(['strptime("%Y", 2021)'], ['E1013: Argument 2: type mismatch, expected string but got number', 'E1174: String required for argument 2'])
-    assert_true(strptime('%Y', '2021') != 0)
-    assert_true(strptime('%Y', '') == 0)
+    assert_notequal(0, strptime('%Y', '2021'))
+    assert_equal(0, strptime('%Y', ''))
   endif
 enddef