test for IO.open as a way of creating an IO instance

Author: alexrford

ql/test/library-tests/frameworks/Files.expected

@@ -34,6 +34,8 @@ ioInstances
 | Files.rb:23:19:23:33 | call to open |
 | Files.rb:24:1:24:40 | ... = ... |
 | Files.rb:24:19:24:40 | call to open |
+| Files.rb:35:1:35:56 | ... = ... |
+| Files.rb:35:13:35:56 | call to open |
 fileReaders
 | Files.rb:7:13:7:32 | call to readlines |
 ioReaders

ql/test/library-tests/frameworks/Files.rb

@@ -30,3 +30,6 @@
 
 # `IO.read("|date")` does not read from a file
 date = IO.read("|date")
+
+# `rand_open` is an `IO` instance
+rand_open = IO.open(IO.sysopen("/dev/random", "r"), "r")