Convert `brew --prefix` test to spec.

Library/Homebrew/test/cmd/--prefix_spec.rb

+describe "brew --prefix", :integration_test do
+  it "prints the Homebrew prefix when no argument is given" do
+    expect { brew "--prefix" }
+      .to output("#{HOMEBREW_PREFIX}\n").to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+
+  it "prints a given Formula's prefix" do
+    expect { brew "--prefix", testball }
+      .to output(%r{#{HOMEBREW_CELLAR}/testball}).to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+end

Library/Homebrew/test/prefix_formula_test.rb

-require "testing_env"
-
-class IntegrationCommandTestPrefixFormula < IntegrationCommandTestCase
-  def test_prefix_formula
-    assert_match "#{HOMEBREW_CELLAR}/testball",
-                 cmd("--prefix", testball)
-  end
-end

Library/Homebrew/test/prefix_test.rb

-require "testing_env"
-
-class IntegrationCommandTestPrefix < IntegrationCommandTestCase
-  def test_prefix
-    assert_equal HOMEBREW_PREFIX.to_s,
-                 cmd("--prefix")
-  end
-end