[gdb/testsuite] Move valgrind-db-attach.{c,exp} to valgrind-bt.{c,exp}

Now that valgrind-db-attach.exp no longer use --db-attach, rename
valgrind-db-attach.{c,exp} to valgrind-bt.{c,exp}.

2018-10-25  Tom de Vries  <tdevries@suse.de>

    * gdb.base/valgrind-db-attach.c: Rename to ...
    * gdb.base/valgrind-bt.c: ... this.
    * gdb.base/valgrind-db-attach.exp: Rename to ...
    * gdb.base/valgrind-bt.exp: ... this.

diff --git a/gdb/testsuite/gdb.base/valgrind-db-attach.c b/gdb/testsuite/gdb.base/valgrind-bt.c
similarity index 100%
rename from gdb/testsuite/gdb.base/valgrind-db-attach.c
rename to gdb/testsuite/gdb.base/valgrind-bt.c

diff --git a/gdb/testsuite/gdb.base/valgrind-db-attach.exp b/gdb/testsuite/gdb.base/valgrind-bt.exp
similarity index 100%
rename from gdb/testsuite/gdb.base/valgrind-db-attach.exp
rename to gdb/testsuite/gdb.base/valgrind-bt.exp