tests: use empty list for g:go_gopls_options

Use an empty list for g:go_gopls_options, because sometime between
2020-08-30 20:07 and 2020-08-31 21:42 some of the tests in
highlight_test started failing in GitHub's CI. All attempts to reproduce
the failures locally have been unsucessful. By outputting the lsp logs
in tests, it became clear that there were some issues with restarting
gopls between test runs and turning off the shared client seems to
resolve the test failures.

scripts/runtest.vim

@@ -20,6 +20,7 @@ if !exists('g:test_verbose')
   let g:test_verbose = 0
 endif
 let g:go_echo_command_info = 0
+let g:go_gopls_options = []
 
 function! s:logmessages() abort
   " Add all messages (usually errors).