vim-patch:9.2.0303: tests: zip plugin tests don't check for warning message properly

Problem:  zip plugin tests may match messages from previous test cases
          when checking for warning message.
Solution: Clear messages at the start of these tests (zeertzjq).

closes: vim/vim#19926

https://github.com/vim/vim/commit/a1f4259e68235e4c8e154842a392d83f3301d2f3
(cherry picked from commit c2d7f6b642)

test/old/testdir/test_plugin_zip.vim

@@ -258,6 +258,7 @@ func Test_zip_fname_evil_path()
   " needed for writing the zip file
   CheckExecutable zip
 
+  messages clear
   call s:CopyZipFile("evil.zip")
   defer delete("X.zip")
   e X.zip
@@ -281,6 +282,7 @@ func Test_zip_fname_evil_path2()
   " needed for writing the zip file
   CheckExecutable zip
 
+  messages clear
   call s:CopyZipFile("evil.zip")
   defer delete("X.zip")
   e X.zip
@@ -303,6 +305,7 @@ func Test_zip_fname_evil_path3()
   " needed for writing the zip file
   CheckExecutable zip
 
+  messages clear
   call s:CopyZipFile("evil.zip")
   defer delete("X.zip")
   e X.zip