Create package spans, nest test/subtests.

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When run as go-test-trace ... without -stdin, we now create spans for each package and the test spans have been moved to be children of them. Subtests are similarly attached as children to the parent test. Here's a screenshot of a trace of go-test-trace net/http/cgi rendered in Jaeger, with full stdlib trace JSON attached below:

go-test-trace-stdlib.json.gz