Add tests for Tab versions

Library/Homebrew/test/fixtures/receipt.json

@@ -16,6 +16,11 @@
   "source": {
       "path": "/usr/local/Library/Taps/hombrew/homebrew-core/Formula/foo.rb",
       "tap": "homebrew/core",
-      "spec": "stable"
+      "spec": "stable",
+      "versions": {
+        "stable": "2.14",
+        "devel": "2.15",
+        "head": "HEAD-0000000"
+      }
   }
 }

Library/Homebrew/test/test_tab.rb

@@ -20,6 +20,11 @@ class TabTests < Homebrew::TestCase
                      "tap" => "homebrew/core",
                      "path" => nil,
                      "spec" => "stable",
+                     "versions" => {
+                       "stable" => "0.10",
+                       "devel" => "0.14",
+                       "head" => "HEAD-1111111",
+                     }
                    })
   end
 
@@ -35,6 +40,9 @@ class TabTests < Homebrew::TestCase
     assert_nil tab.tap
     assert_nil tab.time
     assert_nil tab.HEAD
+    assert_nil tab.stable_version
+    assert_nil tab.devel_version
+    assert_nil tab.head_version
     assert_equal DevelopmentTools.default_compiler, tab.cxxstdlib.compiler
     assert_nil tab.cxxstdlib.type
   end
@@ -105,6 +113,9 @@ class TabTests < Homebrew::TestCase
     assert_equal TEST_SHA1, tab.HEAD
     assert_equal :clang, tab.cxxstdlib.compiler
     assert_equal :libcxx, tab.cxxstdlib.type
+    assert_equal "2.14", tab.stable_version.to_s
+    assert_equal "2.15", tab.devel_version.to_s
+    assert_equal "HEAD-0000000", tab.head_version.to_s
   end
 
   def test_to_json
@@ -119,6 +130,9 @@ class TabTests < Homebrew::TestCase
     assert_equal @tab.HEAD, tab.HEAD
     assert_equal @tab.compiler, tab.compiler
     assert_equal @tab.stdlib, tab.stdlib
+    assert_equal @tab.stable_version, tab.stable_version
+    assert_equal @tab.devel_version, tab.devel_version
+    assert_equal @tab.head_version, tab.head_version
   end
 
   def test_remap_deprecated_options