Convert `brew sh` test to spec.

Library/Homebrew/test/cmd/sh_spec.rb

+describe "brew sh", :integration_test do
+  it "runs a shell with the Homebrew environment" do
+    expect { brew "sh", "SHELL" => which("true") }
+      .to output(/Your shell has been configured/).to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+end

Library/Homebrew/test/sh_test.rb

-require "testing_env"
-
-class IntegrationCommandTestSh < IntegrationCommandTestCase
-  def test_sh
-    assert_match "Your shell has been configured",
-                 cmd("sh", "SHELL" => which("true"))
-  end
-end