Speed up testing with coverage

By running a 'go test -i' on all the packages first the overall runtime is significantly
decreased.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

scripts/test/unit-with-coverage

@@ -1,6 +1,10 @@
 #!/usr/bin/env bash
 set -eu -o pipefail
 
+# install test dependencies once before running tests for each package. This
+# reduces the runtime from 200s down to 23s
+go test -i $@
+
 for pkg in $@; do
     ./scripts/test/unit \
         -cover \