patch 9.0.2058: tests: avoid error when no swap files exist

Problem:  tests: avoid error when no swap files exist
Solution: use unlet! so that no error message is reported
          in case the variable does not exists

When s:GetSwapFileList() does not find any swapfiles, it will return an
empty list []. This means, that the variable 'name' will not be
declared, cause the following unlet command to fail and causing a 1 sec
delay on running the tests.

So let's instead use the :unlet! command which simply skips reporting an
error when the variable given as parameter does not exists.

closes: #13396

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/testdir/runtest.vim

@@ -215,7 +215,7 @@ endfunc
 for name in s:GetSwapFileList()
   call delete(name)
 endfor
-unlet name
+unlet! name
 
 
 " Invoked when a test takes too much time.

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2058,
 /**/
     2057,
 /**/