test: add test for when wmctrl command is not installed

lib/fusuma/plugin/executors/wmctrl_executor.rb

@@ -67,6 +67,8 @@ def search_command(event)
 
         private
 
+        # @return [NilClass] when wmctrl is installed
+        # @raise [NotInstalledError] when wmctrl is not installed
         def must_install!(cmd)
           return if @_installed
 

spec/fusuma/plugin/executors/wmctrl_executor_spec.rb

@@ -22,6 +22,7 @@ module Executors
           record = Events::Records::IndexRecord.new(index: index)
           @event = Events::Event.new(tag: "dummy_detector", record: record)
           @executor = WmctrlExecutor.new
+          allow(@executor).to receive(:must_install!).with("wmctrl").and_return true
           allow(@executor).to receive(:search_command).and_return "dummy command"
         end
 
@@ -74,6 +75,14 @@ module Executors
               .and_return([@current_workspace, @total_workspaces])
           end
 
+
+          context "when wmctrl command is not installed" do
+            before do
+              allow(@executor).to receive(:must_install!).with("wmctrl").and_raise WmctrlExecutor::NotInstalledError
+            end
+            it { expect(@executor.executable?(@event)).to be_falsey }
+          end
+
           context "when workspace: 'prev'" do
             around do |example|
               ConfigHelper.load_config_yml = <<~CONFIG