[gdb/testsuite, tclint] Fix gdb.rust

Running tclint on the test-cases in gdb.rust shows a few problems:
...
modules.exp:37:1: expected braced word or word without substitutions in \
  argument interpreted as expr [command-args]
traits.exp:28:13: expected braced word or word without substitutions in \
  argument interpreted as script [command-args]
...

Fix these.

Tested on aarch64-linux.