[symtab/27831] New test case: gdb.base/add-symbol-file-attach.exp

This commit adds a new test case for bug 27831.  See the contents
of the .exp file for a description of what it's about.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27831
Approved-By: Tom Tromey <tom@tromey.com>