[gdb/testsuite] Fix gdb.base/jit-reader-exec.exp for remote target

With test-case gdb.base/jit-reader-exec.exp and target board
remote-gdbserver-on-localhost (using REMOTE_TARGET_USERNAME) we run into some
failures.

Fix this by adding the missing gdb_remote_download.

Tested on x86_64-linux.

Co-Authored-by: Ivan Tetyushkin <ivan.tetyushkin@syntacore.com>