Isolate rbenv-which tests from any `.ruby-version` file on the system

Having a `.ruby-version` file in any of the parent directories of the
local clone of rbenv could cause the test suite to fail because it
wasn't expecting a local version to be set.

Fixes #533

test/which.bats

@@ -93,7 +93,7 @@ echo HELLO="\$(printf ":%s" "\${hellos[@]}")"
 exit
 SH
 
-  RBENV_HOOK_PATH="$hook_path" IFS=$' \t\n' run rbenv-which anything
+  RBENV_HOOK_PATH="$hook_path" IFS=$' \t\n' RBENV_VERSION=system run rbenv-which anything
   assert_success
   assert_output "HELLO=:hello:ugly:world:again"
 }
@@ -103,6 +103,9 @@ SH
   cat > "${RBENV_ROOT}/version" <<<"1.8"
   create_executable "1.8" "ruby"
 
+  mkdir -p "$RBENV_TEST_DIR"
+  cd "$RBENV_TEST_DIR"
+
   RBENV_VERSION= run rbenv-which ruby
   assert_success "${RBENV_ROOT}/versions/1.8/bin/ruby"
 }