[gdb/testsuite] Fix ERROR in gdb.ctf/funcreturn.exp

[gdb/testsuite] Fix ERROR in gdb.ctf/funcreturn.exp

On openSUSE Tumbleweed (with gcc-12, enabling ctf tests) I run into:
...
ERROR: tcl error sourcing src/gdb/testsuite/gdb.ctf/funcreturn.exp.
ERROR: tcl error code NONE
ERROR: Unexpected arguments: \
  {print v_double_func} \
  {[0-9]+ = {double \(\)} 0x[0-9a-z]+.*} \
  {print double function} \
  }
...

The problem is a curly brace as fourth argument to gdb_test, which errors out
due to recently introduced more strict argument checking in gdb_test.

Fix the error by removing the brace.

Though this fixes the error for me, due to PR29160 I get only FAILs, so I can't
claim proper testing on x86_64-linux.