pkcs7: update test case test_graceful_parsing_failure

Using test_pkcs7.rb as an example invalid input is not reliable because
we may happen to include a valid PKCS#7 PEM block in a heredoc.

Author: rhenium

test/openssl/test_pkcs7.rb

@@ -204,7 +204,7 @@ def test_empty_signed_data_ruby_bug_19974
   end
 
   def test_graceful_parsing_failure #[ruby-core:43250]
-    contents = File.read(__FILE__)
+    contents = "not a valid PKCS #7 PEM block"
     assert_raise(ArgumentError) { OpenSSL::PKCS7.new(contents) }
   end