Split integration tests in fs/net

[0xc0170]

Description (required)

New PR instead of #11703 - Travis could not even see that PR due to unknown reason. As it was previously reviewed, approved and waiting for CI, this can go to CI asap.

Summary of change (What the change is for and why)

Documentation (Details of any document updates required)

---

Pull request type (required)

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

---

Test results (required)

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

---

Reviewers (optional)

---

Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[0xc0170]

Finding another way, the commit sha needs to change.