tests: clean up/beautify all tests (code style / test style)

testcases/t/15-ipc-workspaces.t

@@ -1,7 +1,7 @@
 #!perl
 # vim:ts=4:sw=4:expandtab
 
-use i3test tests => 2;
+use i3test;
 use List::MoreUtils qw(all);
 
 my $i3 = i3("/tmp/nestedcons");
@@ -22,4 +22,4 @@ ok($name_exists, "All workspaces have a name");
 
 }
 
-diag( "Testing i3, Perl $], $^X" );
+done_testing;