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_popup.vim b/src/testdir/test_popup.vim
index 9f727a0..1401e55 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -16,7 +16,7 @@
   if !empty(entered)
     let mth = filter(mth, 'v:val=~"^".entered')
   endif
-  call complete(1, mth) 
+  call complete(1, mth)
   return ''
 endfunc
 
@@ -74,7 +74,7 @@
   call feedkeys("aJu\<f5>\<c-p>l\<c-y>", 'tx')
   call assert_equal(["Jul"], getline(1,2))
   %d
-  
+
   " any-non printable, non-white character: Add this character and
   " reduce number of matches
   call feedkeys("aJu\<f5>\<c-p>l\<c-n>\<c-y>", 'tx')
@@ -96,7 +96,7 @@
   call feedkeys("aJ\<f5>".repeat("\<c-n>",3)."\<c-l>\<esc>", 'tx')
   call assert_equal(["J"], getline(1,2))
   %d
-  
+
   " <c-l> - Insert one character from the current match
   call feedkeys("aJ\<f5>".repeat("\<c-n>",4)."\<c-l>\<esc>", 'tx')
   call assert_equal(["January"], getline(1,2))
@@ -856,7 +856,7 @@
   call term_sendkeys(buf, "jI123456789_\<Esc>")
   call term_sendkeys(buf, "GA\<C-N>")
   call VerifyScreenDump(buf, 'Test_popup_position_04', {'rows': 10})
-  
+
   call term_sendkeys(buf, "\<Esc>u")
   call StopVimInTerminal(buf)
 endfunc