travis: shard tags over separate builds

.travis.yml

@@ -9,6 +9,12 @@ go:
  - 1.10.x
  - master
 
+env:
+ - TAGS=""
+ - TAGS="-tags bounds"
+ - TAGS="-tags noasm"
+ - TAGS="-tags appengine"
+
 matrix:
  fast_finish: true
  allow_failures:
@@ -31,10 +37,7 @@ script:
  - ${TRAVIS_BUILD_DIR}/.travis/check-formatting.sh
  - go get -d -t -v ./...
  - go build -v ./...
- - go test -v ./...
- - go test -a -tags bounds -v ./...
- - go test -a -tags noasm -v ./...
- - go test -a -tags appengine -v ./...
+ - go test -a $TAGS -v ./...
  - if [[ $TRAVIS_SECURE_ENV_VARS = "true" ]]; then bash ./.travis/test-coverage.sh; fi
  - ${TRAVIS_BUILD_DIR}/.travis/check-rand.sh
  # This is run last since it alters the tree.