Move /data directory to /testdata.

[matthewdale]

The Go test tool recommends putting test data in a "testdata" directory:

The go tool will ignore a directory named "testdata", making it available to hold ancillary data needed by the tests.

We do this elsewhere in the repository (e.g. /mongo/options/testdata), but not for the top-level test data directory /data. Move the /data directory to /testdata and update all references.

[qingyang-hu]

What is the purpose of mongo/text.txt?

[benjirewis]

Seems like a good change to me; thanks! Do you know if the system failures in the linked patch are related?

[matthewdale]

@qingyang-hu good find! That mongo/text.txt file was an accidental inclusion. I've removed it.

@benjirewis I don't believe the system failures are related to these changes. However, I rebased on the master branch to make sure this branch has all the latest test fixes. If there are more unexplained test failures, I'll investigate them.

[qingyang-hu]

👍 It looks great!