Comment 1:

The failure is purely deterministic, not a race.
go test x 
has its output redirected to a pipe (and thus available for searching in the go tool)
go test
is not redirected: it is connected directly to the go command's std out and err.
The fix is probably to pass the coverage suffix ("in package xxx" or "") in testmain.go,
and then have the coverage print include the full line. Then 
go test x y
would print output like
ok x coverage: (text from pipe)
ok y coverage: (text from pipe)
but go test
would just let the test output speak for itself and would not re-echo the coverage
information.

Comment 2:

This issue was closed by revision 6d86c14.

Status changed to Fixed.