make+graph/db: add -short helper and skip tests in short mode

In this commit, we add a makefile helper that can be used to add the
"-short" flag to the "go test" command when running unit tests via "make
unit ...".

Tests can then be expanded with a `testing.Short()` check to potentially
skip the test in short mode. This is useful for if a dev wants to
quickly run most of the tests in a package but would like to opt out of
running the longer form tests such as stress tests or rapid generation
tests.

This commit adds this check to two graph/db tests.

graph/db/graph_test.go

@@ -2510,6 +2510,11 @@ func TestFilterKnownChanIDs(t *testing.T) {
 // methods that acquire the cache mutex along with the DB mutex.
 func TestStressTestChannelGraphAPI(t *testing.T) {
 	t.Parallel()
+
+	if testing.Short() {
+		t.Skipf("Skipping test in short mode")
+	}
+
 	ctx := context.Background()
 
 	graph := MakeTestGraph(t)