[Test] gamm: Make test case for GetPoolDenoms (#2240)

Closes: #2200

## What is the purpose of the change
Add test for function GetPoolDenoms 

## Brief Changelog
n/a

## Testing and Verifying
added unit test 

## Documentation and Release Note

  - Does this pull request introduce a new feature or user-facing behavior changes? (no)
  - Is a relevant changelog entry added to the `Unreleased` section in `CHANGELOG.md`? (no)
  - How is the feature or change documented? (not applicable   /   specification (`x/<module>/spec/`)  /  [Osmosis docs repo](https://github.com/osmosis-labs/docs)   /   not documented)

x/gamm/keeper/pool.go

@@ -198,8 +198,14 @@ func (k Keeper) DeletePool(ctx sdk.Context, poolId uint64) error {
 // 	return nil
 // }
 
+// GetPoolDenom retrieves the pool based on PoolId and
+// returns the coin denoms that it holds.
 func (k Keeper) GetPoolDenoms(ctx sdk.Context, poolId uint64) ([]string, error) {
 	pool, err := k.GetPoolAndPoke(ctx, poolId)
+	if err != nil {
+		return nil, err
+	}
+
 	denoms := osmoutils.CoinsDenoms(pool.GetTotalPoolLiquidity(ctx))
 	return denoms, err
 }

x/gamm/keeper/pool_service_test.go

@@ -751,6 +751,45 @@ func (suite *KeeperTestSuite) TestJoinSwapExactAmountInConsistency() {
 	}
 }
 
+func (suite *KeeperTestSuite) TestGetPoolDenom() {
+	// setup pool with denoms
+	suite.FundAcc(suite.TestAccs[0], defaultAcctFunds)
+	poolCreateMsg := balancer.NewMsgCreateBalancerPool(suite.TestAccs[0], defaultPoolParams, defaultPoolAssets, defaultFutureGovernor)
+	_, err := suite.App.GAMMKeeper.CreatePool(suite.Ctx, poolCreateMsg)
+	suite.Require().NoError(err)
+
+	for _, tc := range []struct {
+		desc         string
+		poolId       uint64
+		expectDenoms []string
+		expectErr    bool
+	}{
+		{
+			desc:         "Valid PoolId",
+			poolId:       1,
+			expectDenoms: []string{"bar", "foo"},
+			expectErr:    false,
+		},
+		{
+			desc:         "Invalid PoolId",
+			poolId:       2,
+			expectDenoms: []string{"bar", "foo"},
+			expectErr:    true,
+		},
+	} {
+		suite.Run(tc.desc, func() {
+			denoms, err := suite.App.GAMMKeeper.GetPoolDenoms(suite.Ctx, tc.poolId)
+
+			if tc.expectErr {
+				suite.Require().Error(err)
+			} else {
+				suite.Require().NoError(err)
+				suite.Require().Equal(denoms, tc.expectDenoms)
+			}
+		})
+	}
+}
+
 // func (suite *KeeperTestSuite) TestSetStableSwapScalingFactors() {
 // 	stableSwapPoolParams := stableswap.PoolParams{
 // 		SwapFee: defaultSwapFee,