Make gdb.base/solib-nodir.exp work with --target_board=native-extended-gdbserver

Make gdb.base/solib-nodir.exp work with --target_board=native-extended-gdbserver

Fixes:
 Running .../src/gdb/testsuite/gdb.base/solib-nodir.exp ...
 FAIL: gdb.base/solib-nodir.exp: library loaded

... by using the new "set cwd" command.

gdb/testsuite/ChangeLog:
2017-10-13  Pedro Alves  <palves@redhat.com>
	    Simon Marchi <simon.marchi@polymtl.ca>

	* gdb.base/solib-nodir.exp: Split is_remote and skip_shlib_tests
	calls and add comments.  Skip test if use_gdb_stub is set.
	(top level): Use "set cwd" command instead of "cd" command.