[gdb/testsuite] Fix check-read1 FAIL in attach-many-short-lived-threads.exp

When running test-case gdb.threads/attach-many-short-lived-threads.exp with
check-read1, I ran into:
...
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 1: \
  no new threads (timeout)
...

Fix this by rewriting the gdb_test_multiple call using -lbl and exp_continue.

Tested on x86_64-linux, with make targets check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-14  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/attach-many-short-lived-threads.exp: Read "info threads"
	result in line-by-line fashion.