patch 9.2.0823: tests: Test_clientserver_servlist_list may fail

patch 9.2.0823: tests: Test_clientserver_servlist_list may fail

Problem:  tests: Test_clientserver_servlist_list may fail
Solution: Skip Test_clientserver_serverlist_list for non-gvim GUI builds
          (James McCoy)

Running a GUI build that cannot access the GUI clientserver will fail
like below

    command line..script /build/package/src/testdir/runtest.vim[636]..function RunTheTest[63]..Test_clientserver_serverlist_list[16]..WaitForAssert[2]..<SNR>4_WaitForCommon[11]..<lambda>23 line 1: Pattern 'XVIMTEST' does not match ''
    command line..script /build/package/src/testdir/runtest.vim[636]..function RunTheTest[63]..Test_clientserver_serverlist_list line 18: Expected 'list<string>' but got 'string'
    Caught exception in Test_clientserver_serverlist_list(): Vim(call):E897: List or Blob required @ command line..script /build/package/src/testdir/runtest.vim[636]..function RunTheTest[63]..Test_clientserver_serverlist_list, line 19

closes: #20719

Signed-off-by: James McCoy <jamessan@debian.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>