[gdb/testsuite] Fix build-id check in gdb.python/py-missing-objfile.exp

[gdb/testsuite] Fix build-id check in gdb.python/py-missing-objfile.exp

I recently added two requires in test-case gdb.python/py-missing-objfile.exp:
...
# The following tests assume that the build-ids of binfile and libfile can be
# found in the core file.
require {expect_build_id_in_core_file $binfile}
require {expect_build_id_in_core_file $libfile}
...

However, at the point where the check is done, the files are no longer
available at that location, which makes the require fail.

First, make the problem visible, by making proc expect_build_id_in_core_file
throw an error if the filename argument specifies a non-existing file.

Then, fix test-case gdb.python/py-missing-objfile.exp by moving the calls to
expect_build_id_in_core_file to a point where the files exist.

Tested on x86_64-linux.