Fix various Python import tests.

Formula/enchant.rb

@@ -37,8 +37,10 @@ class Enchant < Formula
   end
 
   def test
+    if build.with? 'python'
       python do
         system python, "-c", "import enchant; d=enchant.Dict('en_US'); print(d.suggest('homebrew'))"
       end
     end
   end
+end

Formula/fontforge.rb

@@ -87,8 +87,12 @@ class Fontforge < Formula
 
   def test
     system "#{bin}/fontforge", "-version"
+    if build.with? 'python'
+      python do
         system python, "-c", "import fontforge"
       end
+    end
+  end
 
   def caveats
     x_caveats = <<-EOS.undent

Formula/libvirt.rb

@@ -61,4 +61,14 @@ class Libvirt < Formula
       end
     end
   end
+
+  test do
+    if build.with? 'python'
+      python do
+        # Testing to import the mod because that is a .so file where linking
+        # can break.
+        system python, '-c', "import libvirtmod"
+      end
+    end
+  end
 end

Formula/shiboken.rb

@@ -49,8 +49,10 @@ class Shiboken < Formula
   end
 
   test do
+    if build.with? 'python'
       python do
         system python, "-c", "import shiboken"
       end
     end
   end
+end