[gdb/testsuite] Fix gdb.threads/pending-fork-event-detach.exp for remote target

[gdb/testsuite] Fix gdb.threads/pending-fork-event-detach.exp for remote target

Fix test-case gdb.threads/pending-fork-event-detach.exp for target board
remote-gdbserver-on-localhost using gdb_remote_download for $touch_file_bin.

Then, fix the test-case for target board remote-stdio-gdbserver with
REMOTE_TMPDIR=~/tmp.remote-stdio-gdbserver by creating $touch_file_path
on target using remote_download, and using the resulting path.

Tested on x86_64-linux.