travis.yml: add TEST_PACKAGES to speed up swarm testing (ethereum#16456)

This commit is meant to allow ecosystem projects such as ethersphere
to minimize CI build times by specifying an environment variable with
the packages to run tests on.

If the environment variable isn't defined the build script will test
all packages so this shouldn't affect the main go-ethereum repository.

.travis.yml

@@ -12,7 +12,7 @@ matrix:
         - sudo chmod 666 /dev/fuse
         - sudo chown root:$USER /etc/fuse.conf
         - go run build/ci.go install
-        - go run build/ci.go test -coverage
+        - go run build/ci.go test -coverage $TEST_PACKAGES
 
     # These are the latest Go versions.
     - os: linux
@@ -24,7 +24,7 @@ matrix:
         - sudo chmod 666 /dev/fuse
         - sudo chown root:$USER /etc/fuse.conf
         - go run build/ci.go install
-        - go run build/ci.go test -coverage
+        - go run build/ci.go test -coverage $TEST_PACKAGES
 
     - os: osx
       go: 1.10.x
@@ -34,7 +34,7 @@ matrix:
         - brew install caskroom/cask/brew-cask
         - brew cask install osxfuse
         - go run build/ci.go install
-        - go run build/ci.go test -coverage
+        - go run build/ci.go test -coverage $TEST_PACKAGES
 
     # This builder only tests code linters on latest version of Go
     - os: linux