Handle failures in temporary_install better

Library/Homebrew/test/test_formula_install.rb

@@ -10,21 +10,20 @@ class InstallTests < Test::Unit::TestCase
   end
 
   def temporary_install f
-    # Brew and install the given formula
+    f.prefix.mkpath
+    keg = Keg.new(f.prefix)
+
     shutup do
       f.brew { f.install }
     end
 
-    # Allow the test to do some processing
+    begin
       yield
-
-    # Remove the brewed formula and double check
-    # that it did get removed. This lets multiple
-    # tests use the same formula name without
-    # stepping on each other.
-    keg=Keg.new f.prefix
+    ensure
       keg.unlink
       keg.uninstall
+    end
+
     assert !keg.exist?
     assert !f.installed?
   end