Fake circuits are created everywhere in the unit tests. This is an attempt at
centralizing a "fake circuit creation" API like fakechans.c does for channel.

This commit introduces fakecircs.c and changes test_relay.c and
test_circpadding.c which were using roughly the same code.

This will allow easier OR circuit creation for the future tests in
test_circuitmux.c

Signed-off-by: David Goulet <dgoulet@torproject.org>