Keeper tests (#590)

* More keeper tests

x/wasm/keeper tests are extended to test various input validation.
Keeper input is validated before passing to the keeper method when used
within wasmd application. We cannot ensure such validation when this
keeper is used outside of wasmd application. To keep it safe, fully
validate keeper methods input.

hackatom.wasm is loaded into memory during initialization to avoid
reading file in each test separately. Once migrated to go 1.16, embed
package should be used instead.

Run goimport on certain files.

Some comments fixed or removed.

* ensure that creator address is not nil

x/wasm/module_test.go

@@ -30,7 +30,6 @@ type testData struct {
 	stakingKeeper stakingkeeper.Keeper
 }
 
-// returns a cleanup function, which must be defered on
 func setupTest(t *testing.T) testData {
 	ctx, keepers := CreateTestInput(t, false, "staking,stargate")
 	cdc := keeper.MakeTestCodec(t)