tests: more options for decoding pcaps

tests: more options for decoding pcaps

Introduce "none", "all" and "failed" options for --decode-pcaps
parameter. Keep "failed" as default to be consistent with current
behaviour. Add missing documentation to test/Makefile and passthrough to
Makefile.

Rationale: running tshark binary takes about 100-150ms and if there are
thousands of pcap files, it takes minutes to decode them. This might not
be desirable if rerunning the tests repeatedly during development.

Type: improvement
Change-Id: Ie033521d51d18b9d499b9bc40fe6eff21c94622d
Signed-off-by: Klement Sekera <[email protected]>