gdb/testsuite: small refactor in selftest-support.exp

Split out the code that makes a copy of the GDB executable ready for
self testing into a new proc.  A later commit in this series wants to
load the GDB executable into GDB (for creating an on-disk debug
index), but doesn't need to make use of the full do_self_tests proc.

There should be no changes in what is tested after this commit.

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