sandbox: add test? method.

Simplify checking if we’re going to sandbox a test with `Sandbox.test?`.

Library/Homebrew/sandbox.rb

@@ -8,6 +8,11 @@ class Sandbox
     OS.mac? && File.executable?(SANDBOX_EXEC)
   end
 
+  def self.test?
+    return false unless available?
+    !ARGV.no_sandbox?
+  end
+
   def self.print_sandbox_message
     unless @printed_sandbox_message
       ohai "Using the sandbox"

Library/Homebrew/test/test_sandbox.rb

@@ -13,6 +13,12 @@ class SandboxTest < Homebrew::TestCase
     @dir.rmtree
   end
 
+  def test_test?
+    ARGV.stubs(:no_sandbox?).returns false
+    assert Sandbox.test?,
+      "Tests should be sandboxed unless --no-sandbox was passed."
+  end
+
   def test_allow_write
     @sandbox.allow_write @file
     @sandbox.exec "touch", @file