Add a_json_string matcher and improve tests

Library/Homebrew/test/cmd/commands_spec.rb

@@ -11,12 +11,11 @@ describe "brew commands", :integration_test do
   end
 
   it "prints a list without headers with the --quiet flag" do
-    expect { brew "commands", "--quiet" }
-      .to_not output(/Built-in commands/).to_stdout
 
     expect { brew "commands", "--quiet" }
       .to be_a_success
       .and not_to_output.to_stderr
+      .and not_to_output(/Built-in commands/).to_stdout
   end
 end
 

Library/Homebrew/test/cmd/info_spec.rb

@@ -14,7 +14,7 @@ describe "brew info", :integration_test do
 
   it "prints as json with the --json=v1 flag" do
     expect { brew "info", "testball", "--json=v1" }
-      .to output(/\{.+testball.+\}/).to_stdout
+      .to output(a_json_string).to_stdout
       .and not_to_output.to_stderr
       .and be_a_success
   end

Library/Homebrew/test/spec_helper.rb

@@ -171,3 +171,14 @@ end
 RSpec::Matchers.define_negated_matcher :not_to_output, :output
 RSpec::Matchers.alias_matcher :have_failed, :be_failed
 RSpec::Matchers.alias_matcher :a_string_containing, :include
+
+RSpec::Matchers.define :a_json_string do
+  match do |actual|
+    begin
+      JSON.parse(actual)
+      true
+    rescue JSON::ParseError
+      false
+    end
+  end
+end