Fix acceptance test assertions for updated plugin remove (elastic#1…

…7122)

This commit updates the acceptance tests to expect messages in the updated
format for removing plugins. See elastic#17030
for change.

qa/acceptance/spec/shared_examples/cli/logstash-plugin/integration_plugin.rb

@@ -75,7 +75,7 @@
       context "trying to uninstall an inner plugin" do
         it "fails to uninstall it" do
           result = logstash.run_command_in_path("bin/logstash-plugin uninstall logstash-input-rabbitmq")
-          expect(result.stderr).to match(/is already provided by/)
+          expect(result.stderr).to include("The plugin `logstash-input-rabbitmq` is provided by 'logstash-integration-rabbitmq' so it can't be removed individually")
         end
       end
     end

qa/acceptance/spec/shared_examples/cli/logstash-plugin/uninstall.rb

@@ -33,7 +33,7 @@
     context "when the plugin isn't installed" do
       it "fails to uninstall it" do
         result = logstash.run_command_in_path("bin/logstash-plugin uninstall logstash-filter-qatest")
-        expect(result.stderr).to match(/This plugin has not been previously installed/)
+        expect(result.stderr).to include("The plugin `logstash-filter-qatest` has not been previously installed")
       end
     end