Fix three test failures with extended remote targets

Fix three test failures with extended remote targets

This commit fixes three gdb.base/attach.exp failures when using
extended remote targets.  The failures occurred because GDB now
locates and loads files when attaching on remote targets if the
remote target supports qXfer:exec-file:read; the filenames were
shown but with "target:" prefixes which the test has been updated
to handle.

gdb/testsuite/ChangeLog:

	* gdb.base/attach.exp: Fix three extended remote failures.