tests: add test for the for_window config directive, use separate config for tests

This test increases code coverage to 60.7%

testcases/complete-run.pl

@@ -28,7 +28,7 @@ my $result = GetOptions(
     "coverage-testing" => \$coverage_testing
 );
 
-my $i3cmd = "export DISPLAY=:0; exec " . abs_path("../i3") . " -V -d all --disable-signalhandler -c " . abs_path("../i3.config");
+my $i3cmd = "export DISPLAY=:0; exec " . abs_path("../i3") . " -V -d all --disable-signalhandler -c i3-test.config";
 
 # 1: get a list of all testcases
 my @testfiles = @ARGV;