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/sql_migration_test.go

@@ -1177,6 +1177,10 @@ func assertResultState(t *testing.T, sql *SQLStore, expState dbState) {
 func TestMigrateGraphToSQLRapid(t *testing.T) {
 	t.Parallel()
 
+	if testing.Short() {
+		t.Skipf("skipping test in short mode")
+	}
+
 	dbFixture := NewTestDBFixture(t)
 
 	rapid.Check(t, func(rt *rapid.T) {