sanitycheck: support listing test cases

Parse all yaml file and create a list of declared testcases. This does
list the individual tests inside test projects, not only the projects
containing the tests, for example:

$ sanitycheck --list-tests  -T tests/net/socket/
 - net.socket.udp.send_recv_2_sock
 - net.socket.udp.v4_sendto_recvfrom
 - net.socket.udp.v6_sendto_recvfrom
 - net.socket.udp.v4_bind_sendto
 - net.socket.udp.v6_bind_sendto
 - net.socket.getaddrinfo_ok
 - net.socket.getaddrinfo_no_host
 - net.socket.tcp.v4_send_recv
 - net.socket.tcp.v6_send_recv
 - net.socket.tcp.v4_sendto_recvfrom
 - net.socket.tcp.v6_sendto_recvfrom
 - net.socket.tcp.v4_sendto_recvfrom_null_dest
 - net.socket.tcp.v6_sendto_recvfrom_null_dest
13 total.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>