snap: Revert to "mage test" because "mage check" fails

"mage check", which runs "go test", gofmt, "go vet", and
"go test -race", fails one way or another especially on
on non-amd64 architectures, and sometimes even on amd64 too.

Partially reverting my undocumented change in commit 294c0f8.

Fixes #4578

snapcraft.yaml

@@ -29,7 +29,7 @@ parts:
       export PATH=$GOPATH/bin:$PATH
       cd $GOPATH/src/github.com/gohugoio/hugo
       go get github.com/magefile/mage
-      mage -v vendor check
+      mage -v vendor test
     build: |
       export GOPATH=$(dirname $SNAPCRAFT_PART_INSTALL)/go
       export PATH=$GOPATH/bin:$PATH