editorconfig: set trim_trailing_whitespace = false for src/testdir/test*.vim

closes: #16220

Signed-off-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/.editorconfig b/.editorconfig
index a586af4..7dc0cfb 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -23,3 +23,7 @@
 [runtime/doc/**.txt]
 # It can mess up some documentation by trying to strip trailing whitespaces
 trim_trailing_whitespace = false
+
+[src/testdir/test*.vim]
+# Some tests need trailing whitespaces, for example `set showbreak=>>\ `
+trim_trailing_whitespace = false