test/unpack_strategy: Handle svn unzip unavailable

Skip those tests that require svn or unzip.

Library/Homebrew/test/spec_helper.rb

@@ -85,7 +85,11 @@ RSpec.configure do |config|
   end
 
   config.before(:each, :needs_svn) do
-    skip "Requires subversion." unless which "svn"
+    skip "subversion not installed." unless which "svn"
+  end
+
+  config.before(:each, :needs_unzip) do
+    skip "unzip not installed." unless which("unzip")
   end
 
   config.around(:each) do |example|

Library/Homebrew/test/unpack_strategy/jar_spec.rb

@@ -1,6 +1,6 @@
 require_relative "shared_examples"
 
-describe UnpackStrategy::Jar do
+describe UnpackStrategy::Jar, :needs_unzip do
   let(:path) { TEST_FIXTURE_DIR/"test.jar" }
 
   include_examples "UnpackStrategy::detect"

Library/Homebrew/test/unpack_strategy/subversion_spec.rb

@@ -1,6 +1,6 @@
 require_relative "shared_examples"
 
-describe UnpackStrategy::Subversion do
+describe UnpackStrategy::Subversion, :needs_svn do
   let(:repo) { mktmpdir }
   let(:working_copy) { mktmpdir }
   let(:path) { working_copy }

Library/Homebrew/test/unpack_strategy/zip_spec.rb

@@ -4,7 +4,10 @@ describe UnpackStrategy::Zip do
   let(:path) { TEST_FIXTURE_DIR/"cask/MyFancyApp.zip" }
 
   include_examples "UnpackStrategy::detect"
+
+  context "when unzip is available", :needs_unzip do
     include_examples "#extract", children: ["MyFancyApp"]
+  end
 
   context "when ZIP archive is corrupted" do
     let(:path) {