Commonalise parser test variables.

Move various string constants to package scope to avoid redefining them
in each test function.
Minor formatting tweaks.
Show number of tests passed on 'go test'.

Display the number of tests that passed as well as the total when 'go
test' is run, and also show the pass rate in %.
Fix header params in headers with non-<> URIs

Correctly parse to/from/contact headers with URIs not quoted by <, > and
which have header parameters following the URI.

Example (broken before this fix):
To: joe@bloggs.com;foo=bar

Non-example (was never broken):
To: <joe@bloggs.com>;foo=bar
Add last few Contact: header tests
Handle multiple to/from headers in tests better

Panic instead of suppressing the unexpected headers.