[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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+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.
+
 2020-05-25  Tom de Vries  <tdevries@suse.de>
 
 	PR testsuite/26031

gdb/testsuite/lib/jit-elf-helpers.exp

@@ -99,7 +99,7 @@ proc compile_and_download_n_jit_so {jit_solib_basename jit_solib_srcfile count}
 	if { [gdb_compile_shlib ${jit_solib_srcfile} ${binfile} \
 		  $options] != "" } {
 	    set f [file tail $binfile]
-	    untested "failed to compile shared library $binfile"
+	    untested "failed to compile shared library $f"
 	    return -1
 	}