gdb/testsuite: replace $testfile with $binfile in one case

For *reasons* I was hacking on gdb.base/foll-vfork.exp and wanted to
change the name of the binary that was created.  Should be easy, I
adjusted the global $binfile variable .... but that didn't work.

In one place the script uses $testfile instead of $binfile.

Fixed this to use $binfile, now I can easily change the name of the
generated binary, and the test still works.

There's no change in what is tested after this commit.