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_increment.vim b/src/testdir/test_increment.vim
index beacb0a..c367f2b 100644
--- a/src/testdir/test_increment.vim
+++ b/src/testdir/test_increment.vim
@@ -704,7 +704,7 @@
 " Text:
 "   1 23
 "   4 56
-" 
+"
 " Expected:
 "   1) f2 Ctrl-V jl <ctrl-a>, repeat twice afterwards with .
 "   1 26