Convert MPIRequirement test to spec.

Library/Homebrew/test/mpi_requirement_spec.rb

+require "requirements/mpi_requirement"
+
+describe MPIRequirement do
+  describe "::new" do
+    subject { described_class.new(*(wrappers + tags)) }
+    let(:wrappers) { [:cc, :cxx, :f77] }
+    let(:tags) { [:optional, "some-other-tag"] }
+
+    it "untangles wrappers and tags" do
+      expect(subject.lang_list).to eq(wrappers)
+      expect(subject.tags).to eq(tags)
+    end
+  end
+end

Library/Homebrew/test/mpi_requirement_test.rb

-require "testing_env"
-require "requirements/mpi_requirement"
-
-class MPIRequirementTests < Homebrew::TestCase
-  def test_initialize_untangles_tags_and_wrapper_symbols
-    wrappers = [:cc, :cxx, :f77]
-    tags = [:optional, "some-other-tag"]
-    dep = MPIRequirement.new(*wrappers + tags)
-    assert_equal wrappers, dep.lang_list
-    assert_equal tags, dep.tags
-  end
-end