patch 9.0.0979: ch_log() text can be hard to find in the log file

Problem:    ch_log() text can be hard to find in the log file.
Solution:   Prepend "ch_log()" to the text.
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index a99497d..438db8e 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -2293,7 +2293,8 @@
   call ch_log('%s%s')
   call ch_logfile('')
   let text = readfile('Xlog')
-  call assert_match("hello there", text[1])
+  call assert_match("start log session", text[0])
+  call assert_match("ch_log(): hello there", text[1])
   call assert_match("%s%s", text[2])
   call mkdir("Xchlogdir1", 'D')
   call assert_fails("call ch_logfile('Xchlogdir1')", 'E484:')