* internal/testlog: print file+line number of log call in test log

This changes the unit test logger to print the actual file and line
number of the logging call instead of "testlog.go:44".

Output of 'go test -v -run TestServerListen ./p2p' before this change:

    === RUN   TestServerListen
    --- PASS: TestServerListen (0.00s)
        testlog.go:44: DEBUG[01-08|15:16:31.651] UDP listener up         addr=127.0.0.1:62678
        testlog.go:44: DEBUG[01-08|15:16:31.651] TCP listener up         addr=127.0.0.1:62678
        testlog.go:44: TRACE[01-08|15:16:31.652] Accepted connection     addr=127.0.0.1:62679

And after:

    === RUN   TestServerListen
    --- PASS: TestServerListen (0.00s)
        server.go:868: DEBUG[01-08|15:25:35.679] TCP listener up         addr=127.0.0.1:62712
        server.go:557: DEBUG[01-08|15:25:35.679] UDP listener up         addr=127.0.0.1:62712
        server.go:912: TRACE[01-08|15:25:35.680] Accepted connection     addr=127.0.0.1:62713

* internal/testlog: document use of t.Helper