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

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>