chore: avoid test flakiness in v2 API tests caused by WaitTillChain (#13033)

Avoid test flakiness in v2 API tests caused by `WaitTillChain`

Use a height that is sufficiently behind the target height to avoid race
conditions in WaitTillChain that could result in tests running earlier
than state change is reflected across the system.

Observed in:
* https://github.com/filecoin-project/lotus/actions/runs/14401881056/job/40389322945?pr=13026

Author: masih

itests/api_v2_test.go

@@ -179,8 +179,11 @@ func TestAPIV2_ThroughRPC(t *testing.T) {
 					mockF3.latestCert = nil
 					mockF3.latestCertErr = nil
 				},
-				request:            `{"jsonrpc":"2.0","method":"Filecoin.ChainGetTipSet","params":[{"height":{"at":20}}],"id":1}`,
-				wantTipSet:         tipSetAtHeight(20),
+				// Lookup a height that is sufficiently behind the epoch at WaitTillChain + a
+				// little bit further back to avoid race conditions of WaitTillChain
+				// implementation which can cause flaky tests.
+				request:            `{"jsonrpc":"2.0","method":"Filecoin.ChainGetTipSet","params":[{"height":{"at":15}}],"id":1}`,
+				wantTipSet:         tipSetAtHeight(15),
 				wantResponseStatus: http.StatusOK,
 			},
 			{