fix: add WaitForNextBlock to prevent flaky testnode tests (#7046)

## Summary
- Add missing `cctx.WaitForNextBlock()` calls after
`testnode.NewNetwork` in three tests that were susceptible to flaking
when the node hadn't produced its first block
- Affected tests: `TestClaimRewardsAfterFullUndelegation`,
`StandardSDKIntegrationTestSuite.SetupSuite`,
`TestTimeInPrepareProposalContext`
- Matches the pattern already used by non-flaky tests like
`integration_test.go` and `send_to_self_test.go`

Closes #7045

## Test plan
- [x] `TestClaimRewardsAfterFullUndelegation` passes 3/3 runs locally
with `-count=3`
- [ ] `test / test-short` CI job passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rootulp

app/test/claim_test.go

@@ -29,6 +29,7 @@ func TestClaimRewardsAfterFullUndelegation(t *testing.T) {
 	accounts := testfactory.GenerateAccounts(2)
 	config := testnode.DefaultConfig().WithFundedAccounts(accounts...)
 	cctx, _, _ := testnode.NewNetwork(t, config)
+	require.NoError(t, cctx.WaitForNextBlock())
 
 	txClient, err := testnode.NewTxClientFromContext(cctx)
 	require.NoError(t, err)

app/test/prepare_proposal_context_test.go

@@ -35,6 +35,7 @@ func TestTimeInPrepareProposalContext(t *testing.T) {
 	vestAccName := "vesting"
 	cfg := testnode.DefaultConfig().WithFundedAccounts(sendAccName)
 	cctx, _, _ := testnode.NewNetwork(t, cfg)
+	require.NoError(t, cctx.WaitForNextBlock())
 	enc := encoding.MakeConfig(app.ModuleEncodingRegisters...)
 
 	type test struct {

app/test/std_sdk_test.go

@@ -66,6 +66,7 @@ func (s *StandardSDKIntegrationTestSuite) SetupSuite() {
 	s.accounts = testfactory.GenerateAccounts(35)
 	s.cfg = testnode.DefaultConfig().WithFundedAccounts(s.accounts...)
 	s.cctx, _, _ = testnode.NewNetwork(t, s.cfg)
+	require.NoError(t, s.cctx.WaitForNextBlock())
 	s.ecfg = encoding.MakeConfig(app.ModuleEncodingRegisters...)
 }