gdb/testsuite: attach to i386 process stopped in vDSO

gdb/testsuite: attach to i386 process stopped in vDSO

Fedora GDB has carried around a patch for a while which tested
attaching to an i386 process which is stopped within the vDSO library
region.  Apparently, at some point in the distant past there was an
issue finding symbol information for this region in this situation.

I'm struggling to track down the precise details of what the original
bug was, however, acquiring symbol information for the vDSO region is
different than for "normal" shared libraries -- the vDSO information
is synthesised within GDB during the attach / inferior creation
process -- so it's not unreasonable to imagine that there could be a
bug specifically in this area of GDB which wouldn't impact "normal"
shared libraries.

I looked for references to vDSO in our testsuite and couldn't find
any tests that looked like they did the same sort of thing, so I'd
like to propose adding this test to our testsuite.

It's a pretty simple test, and doesn't take long to run, so the cost
of adding this is not huge.

Approved-By: Tom Tromey <tom@tromey.com>