File descriptor 99 is reserved for internal use

It should not, by convention, be used by any test cases; its opening here
will (presumably, at least) prevent the shell from automatically allocating
it for any file operations and subshells.

src/expect-core

@@ -22,6 +22,9 @@
 [ -z $__INC_EXPECT_CORE ] || return
 __INC_EXPECT_CORE=1
 
+# reserved for our uses
+exec 99<>/dev/null
+
 
 ##
 # Shorthand for bailing out on unrecognized clauses