[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.