[gdb/testsuite] Fix var use in compile_and_download_n_jit_so

[gdb/testsuite] Fix var use in compile_and_download_n_jit_so

In commit 1b59ca1cf1 "[gdb/testsuite] Fix tcl error in jit-elf-helpers.exp", I
introduced a variable f in compile_and_download_n_jit_so, to be used in the
untested message, but actually variable binfile was used instead:
...
+           set f [file tail $binfile]
+           untested "failed to compile shared library $binfile"
...

Fix this by using $f in the untested message.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-25  Tom de Vries  <tdevries@suse.de>

	* lib/jit-elf-helpers.exp (compile_and_download_n_jit_so): Use $f
	instead of $binfile in the untested message.