patch 8.2.2741: Vim9: Partial call does not check right arguments

Problem:    Vim9: Partial call does not check right arguments.
Solution:   Adjust the offset for whether the partial is before or after the
            arguments. (closes #8091)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index d48cab4..85830a3 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -796,6 +796,8 @@
     assert_equal('anything', Callback())
     assert_equal('anything', Callback(1))
     assert_equal('anything', Callback('a', 2))
+
+    assert_equal('xyz', ((a: string): string => a)('xyz'))
   END
   CheckDefAndScriptSuccess(lines)