tests: check that -f -p attaches to threads properly

Check that -f -p attaches and follows all threads of the given pid.

* tests/attach-f-p.c: New file.
* tests/attach-f-p.test: New test.
* tests/.gitignore: Add attach-f-p.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(attach_f_p_LDADD): New variable.
(TESTS): Add attach-f-p.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index 690242e..c5dd3cb 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -8,6 +8,7 @@
 _newselect
 adjtimex
 aio
+attach-f-p
 attach-p-cmd-cmd
 attach-p-cmd-p
 bpf