go/types: add a race test

[griesemer]

Reminder issue: go/types can be invoked concurrently and while the design is such that this should be ok (we know it's working), it would be good to have a test for it.

See also #32154 for a most recent failure due to a race condition.

(A test can be added to (*tester).raceTest in cmd/dist/test.go.)

[dmitshur]

#36605 (comment) is relevant. /cc @matloob

[dmitshur]

/cc @findleyr @stamblerre @heschik

[bcmills]

This may be relevant to #37602 and #37690.

[findleyr]

This was done in #47729