patch 8.2.1724: Vim9: assignment tests spread out

Problem:    Vim9: assignment tests spread out.
Solution:   Create new test file for assignment tests.

src/testdir/test_vim9_expr.vim

@@ -1687,18 +1687,6 @@ def Test_expr7_lambda_vim9script()
 	->map({_, v -> synIDattr(v, 'name')})->len()})
   END
   CheckScriptSuccess(lines)
-
-  # check if assign a lambda to a variable which type is func or any.
-  lines =<< trim END
-      vim9script
-      let FuncRef = {->123}
-      assert_equal(123, FuncRef())
-      let FuncRef_Func: func = {->123}
-      assert_equal(123, FuncRef_Func())
-      let FuncRef_Any: any = {->123}
-      assert_equal(123, FuncRef_Any())
-  END
-  CheckScriptSuccess(lines)
 enddef
 
 def Test_epxr7_funcref()