pack/testpack: fix build
pack/testpack: fix build

There were two func Strings defined. Revert to the previous, which called it "debug" to go along with DebugPack

All tests pass again.

TBR=p

Change-Id: I2b915c891b2ec8fa961d9602e76b5b589a60c9fc
Reviewed-on: https://upspin-review.git.corp.google.com/1294
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin