patch 9.0.1257: code style is not check in test scripts

Problem:    Code style is not check in test scripts.
Solution:   Add basic code style check for test files.
diff --git a/src/testdir/test_utf8.vim b/src/testdir/test_utf8.vim
index 9ad7828..16bf60d 100644
--- a/src/testdir/test_utf8.vim
+++ b/src/testdir/test_utf8.vim
@@ -1,5 +1,5 @@
 " Tests for Unicode manipulations
- 
+
 source check.vim
 source view_util.vim
 source screendump.vim
@@ -107,7 +107,7 @@
 
   let save_encoding = &encoding
   set encoding=latin1
-  
+
   let lres = str2list(s, 1)
   let sres = list2str(l, 1)
   call assert_equal([65, 66, 67], str2list("ABC"))
@@ -123,7 +123,7 @@
 func Test_screenchar_utf8()
   new
 
-  " 1-cell, with composing characters 
+  " 1-cell, with composing characters
   call setline(1, ["ABC\u0308"])
   redraw
   call assert_equal([0x0041], screenchars(1, 1))
@@ -133,7 +133,7 @@
   call assert_equal("B", screenstring(1, 2))
   call assert_equal("C\u0308", screenstring(1, 3))
 
-  " 2-cells, with composing characters 
+  " 2-cells, with composing characters
   let text = "\u3042\u3044\u3046\u3099"
   call setline(1, text)
   redraw