[gdb/testsuite] Fix gdb.base/fork-no-detach-follow-child-dlopen.exp for remote target

Fix test-case gdb.base/fork-no-detach-follow-child-dlopen.exp for target board
remote-gdbserver-on-localhost.exp by using gdb_download_shlib and gdb_locate_shlib.

Tested on x86_64-linux.