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_scroll_opt.vim b/src/testdir/test_scroll_opt.vim
index 58344f9..c146c01 100644
--- a/src/testdir/test_scroll_opt.vim
+++ b/src/testdir/test_scroll_opt.vim
@@ -271,18 +271,18 @@
   call term_sendkeys(buf, ":set scrolloff=1\<CR>")
   call term_sendkeys(buf, "10|\<C-E>")
   call VerifyScreenDump(buf, 'Test_smooth_long_6', {})
-  
+
   " 'scrolloff' set to 1, scrolling down, cursor moves screen line up
   call term_sendkeys(buf, "\<C-E>")
   call term_sendkeys(buf, "gjgj")
   call term_sendkeys(buf, "\<C-Y>")
   call VerifyScreenDump(buf, 'Test_smooth_long_7', {})
-  
+
   " 'scrolloff' set to 2, scrolling up, cursor moves screen line down
   call term_sendkeys(buf, ":set scrolloff=2\<CR>")
   call term_sendkeys(buf, "10|\<C-E>")
   call VerifyScreenDump(buf, 'Test_smooth_long_8', {})
-  
+
   " 'scrolloff' set to 2, scrolling down, cursor moves screen line up
   call term_sendkeys(buf, "\<C-E>")
   call term_sendkeys(buf, "gj")
@@ -320,7 +320,7 @@
   call term_sendkeys(buf, "3Gzt")
   call term_sendkeys(buf, "\<C-E>j")
   call VerifyScreenDump(buf, 'Test_smooth_long_15', {})
-  
+
   call StopVimInTerminal(buf)
 endfunc
 
@@ -335,7 +335,7 @@
   call writefile(lines, 'XSmoothOneLong', 'D')
   let buf = RunVimInTerminal('-S XSmoothOneLong', #{rows: 6, cols: 40})
   call VerifyScreenDump(buf, 'Test_smooth_one_long_1', {})
-  
+
   call term_sendkeys(buf, "\<C-E>")
   call VerifyScreenDump(buf, 'Test_smooth_one_long_2', {})
 
@@ -357,7 +357,7 @@
   call writefile(lines, 'XSmoothLongShowbreak', 'D')
   let buf = RunVimInTerminal('-S XSmoothLongShowbreak', #{rows: 6, cols: 40})
   call VerifyScreenDump(buf, 'Test_smooth_long_showbreak_1', {})
-  
+
   call term_sendkeys(buf, "\<C-E>")
   call VerifyScreenDump(buf, 'Test_smooth_long_showbreak_2', {})
 
@@ -465,7 +465,7 @@
   call NewWindow(10, 20)
   setl smoothscroll wrap
   setl scrolloff=3
-  
+
   " 120 chars are 6 screen lines
   call setline(1, "abcdefghijklmnopqrstABCDEFGHIJKLMNOPQRSTabcdefghijklmnopqrstABCDEFGHIJKLMNOPQRSTabcdefghijklmnopqrstABCDEFGHIJKLMNOPQRST")
   call setline(2, "below")