test: import file name for issue19028

The pattern in NNN.dir directories is that if we have a.go,
the other files import "./a". For gc it happens to work to use a path,
but not for gofrontend. Better to be consistent.

Change-Id: I2e023cbf6bd115f9fb77427b097b0ff9b9992f17
Reviewed-on: https://go-review.googlesource.com/c/go/+/278113


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>