Convert `brew config` test to spec.

Library/Homebrew/test/cmd/config_spec.rb

+describe "brew config", :integration_test do
+  it "prints information about the current Homebrew configuration" do
+    expect { brew "config" }
+      .to output(/HOMEBREW_VERSION: #{HOMEBREW_VERSION}/).to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+end

Library/Homebrew/test/config_test.rb

-require "testing_env"
-
-class IntegrationCommandTestConfig < IntegrationCommandTestCase
-  def test_config
-    assert_match "HOMEBREW_VERSION: #{HOMEBREW_VERSION}",
-                 cmd("config")
-  end
-end