Add test for flags.

License: MIT
Signed-off-by: Zander Mackie <zmackie@gmail.com>

flags/flags_test.go

+package flags
+
+import (
+	"os"
+	"testing"
+)
+
+// This variable is initialized before flags init(), so we export the ENV variable here.
+var _lowMemOn = lowMemOn()
+
+func lowMemOn() error {
+	os.Setenv("IPFS_LOW_MEM", "true")
+	return nil
+}
+
+func TestLowMemMode(t *testing.T) {
+	if !LowMemMode {
+		t.Fatal("LowMemMode does not turn on even with 'IPFS_LOW_MEM' ENV variable set.")
+	}
+}