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_popupwin_textprop.vim b/src/testdir/test_popupwin_textprop.vim
index 3472b40..80b5a88 100644
--- a/src/testdir/test_popupwin_textprop.vim
+++ b/src/testdir/test_popupwin_textprop.vim
@@ -21,7 +21,7 @@
 		\ bufnr: bufnr('%'),
 		\ })
 	let winid = popup_create('the text', #{
-	      \ pos: 'botleft', 
+	      \ pos: 'botleft',
 	      \ textprop: 'popupMarker',
 	      \ border: [],
 	      \ padding: [0,1,0,1],
@@ -67,25 +67,25 @@
 		\ type: 'popupMarker',
 		\ })
 	let winid = popup_create('bottom left', #{
-	      \ pos: 'botleft', 
+	      \ pos: 'botleft',
 	      \ textprop: 'popupMarker',
 	      \ textpropwin: win_getid(),
 	      \ padding: [0,1,0,1],
 	      \ })
 	let winid = popup_create('bottom right', #{
-	      \ pos: 'botright', 
+	      \ pos: 'botright',
 	      \ textprop: 'popupMarker',
 	      \ border: [],
 	      \ padding: [0,1,0,1],
 	      \ })
 	let winid = popup_create('top left', #{
-	      \ pos: 'topleft', 
+	      \ pos: 'topleft',
 	      \ textprop: 'popupMarker',
 	      \ border: [],
 	      \ padding: [0,1,0,1],
 	      \ })
 	let winid = popup_create('top right', #{
-	      \ pos: 'topright', 
+	      \ pos: 'topright',
 	      \ textprop: 'popupMarker',
 	      \ padding: [0,1,0,1],
 	      \ })
@@ -127,14 +127,14 @@
 		\ type: 'popupMarker',
 		\ })
 	let winid = popup_create('bottom left', #{
-	      \ pos: 'botleft', 
+	      \ pos: 'botleft',
 	      \ line: -1,
 	      \ col: 2,
 	      \ textprop: 'popupMarker',
 	      \ padding: [0,1,0,1],
 	      \ })
 	let winid = popup_create('bottom right', #{
-	      \ pos: 'botright', 
+	      \ pos: 'botright',
 	      \ line: -1,
 	      \ col: -2,
 	      \ textprop: 'popupMarker',
@@ -142,7 +142,7 @@
 	      \ padding: [0,1,0,1],
 	      \ })
 	let winid = popup_create('top left', #{
-	      \ pos: 'topleft', 
+	      \ pos: 'topleft',
 	      \ line: 1,
 	      \ col: 2,
 	      \ textprop: 'popupMarker',
@@ -150,7 +150,7 @@
 	      \ padding: [0,1,0,1],
 	      \ })
 	let winid = popup_create('top right', #{
-	      \ pos: 'topright', 
+	      \ pos: 'topright',
 	      \ line: 1,
 	      \ col: -2,
 	      \ textprop: 'popupMarker',