[gdb/testsuite] Fix PATH warning in gdb.rust/traits.exp

[gdb/testsuite] Fix PATH warning in gdb.rust/traits.exp

When running the rust test-cases with release 1.36.0 and LLVM version 7.0, I
run into:
...
(gdb) UNTESTED: gdb.rust/traits.exp: could not read \
  outputs/gdb.rust/traits/traits with readelf
PATH: gdb.rust/traits.exp: could not read \
  outputs/gdb.rust/traits/traits with readelf
...

Fix the PATH warning by printing [file tail $binfile] instead $binfile.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-09-14  Tom de Vries  <tdevries@suse.de>

	* gdb.rust/traits.exp: Fix PATH warning.