Refactor requirement specs.

Library/Homebrew/test/os_requirement_spec.rb

@@ -1,18 +0,0 @@
-require "requirements/linux_requirement"
-require "requirements/macos_requirement"
-
-describe LinuxRequirement do
-  describe "#satisfied?" do
-    it "returns true if OS is Linux" do
-      expect(subject.satisfied?).to eq(OS.linux?)
-    end
-  end
-end
-
-describe MacOSRequirement do
-  describe "#satisfied?" do
-    it "returns true if OS is macOS" do
-      expect(subject.satisfied?).to eq(OS.mac?)
-    end
-  end
-end

Library/Homebrew/test/requirements/linux_requirement_spec.rb

@@ -0,0 +1,11 @@
+require "requirements/linux_requirement"
+
+describe LinuxRequirement do
+  subject(:requirement) { described_class.new }
+
+  describe "#satisfied?" do
+    it "returns true on Linux" do
+      expect(requirement.satisfied?).to eq(OS.linux?)
+    end
+  end
+end

Library/Homebrew/test/requirements/macos_requirement_spec.rb

@@ -0,0 +1,11 @@
+require "requirements/macos_requirement"
+
+describe MacOSRequirement do
+  subject(:requirement) { described_class.new }
+
+  describe "#satisfied?" do
+    it "returns true on macOS" do
+      expect(requirement.satisfied?).to eq(OS.mac?)
+    end
+  end
+end