tests-mbed_hal-rtc patch to match IAR compilation issue

[jeromecoutant]

Summary of changes

Issue detected in #12932

Whole thing seems like a random compiler bug. At the specific error point you could replace 100s with seconds(100)?

@kjbracey-arm
@MarceloSalazar

Impact of changes

None for ARM and GCC

Test compilation is now OK with IAR

Migration actions required

Documentation

---

Pull request type

[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

[] 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

---

[kjbracey]

"Fix" is totally random - 400s is used in the same line.

At least stick a comment next to it, otherwise someone might change it back, and there's no CI to warn them.

(And who knows when the same problem might occur elsewhere? I guess it's reassuring that it hasn't so far - there's a lot of other time_points in the real tree. Wonder what the necessary conditions are. I'd report this to IAR if I were you. Along with the nontrivial lambda thing - which isn't technically a bug, but is distinctly suboptimal for embedded optimisation.)

[ciarmcom]

@jeromecoutant, thank you for your changes.
@ARMmbed/mbed-os-hal @ARMmbed/mbed-os-test @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Travis restarted, this will go into CI

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 4 of 4 test jobs passed
Build number : 1
Build artifacts

[mergify]

This PR does not contain release version label after merging.