[gdb/testsuite] Fix gdb.base/solib-vanish.exp for remote target

When running test-case gdb.base/solib-vanish.exp with target board
remote-gdbserver-on-localhost (using REMOTE_TARGET_USERNAME) we run into some
failures.

Fix these by adding the missing gdb_load_shlib.

Tested on x86_64-linux.

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