Refactor Logs APIs and implementation

[prashantv]

TestResult.Logs() now returns a list of structured entries for logs.

Most callers will likely rely on Logs.String() to get a concatenated string
of all logs, in a format similar to go test. However, tests can also get a
list of messages, or work with the Log entry directly.

The log entry includes the message, caller information, and the testing.TB
function that was used to log. In the future, we can include caller PCs if
there's use-cases, but PCs are not exposed due to the complexity of
merging caller PCs across the log caller, and possibly a cleanup caller.