[RFC] Enable test coverage infrastructure for end-2-end, functional and unittests

@sasdf is already working on this.

Since Jul 16, 2025.

### Description

Test coverage metrics are an important software quality metric enabling the identification of test coverage gaps across, unit, functional and end-2-end test levels.

Coverage reports of released artifacts are also required for Common Criteria certification.

## Scope

This issue proposes the enablement of test coverage reports for the following components.

### Artifacts

* `ROM`
* `ROM_EXT`
* `cryptolib`

### Test Environments

* `unittest`
* `functest`
* `end-2-end`
* `cryptotest`

## Implementation Details

There two different approaches that can be followed to implement coverage reports:

1. Compile-time instrumentation
2. Trace generation from Simulation or Emulation environments.

Simulation traces are out of scope at the moment given performance limitations. `ROM` end-2-end test cases take hours to run in simulation environments.

Software based emulation (e.g. QEMU, Renode) are not an option at the moment due to lack of RTL/silicon feature parity.

The only reasonable option at the moment is to implement compile time instrumentation. This issue recommends using https://github.com/lowRISC/opentitan/pull/3080 as a reference for an implementation supported by Bazel.

## Tasks

- [ ] Implement test coverage instrumentation for `unittests`.
- [ ] Port https://github.com/lowRISC/opentitan/pull/3080 to Bazel to enable coverage for
`functest and `end-2-end`.
- [ ] Instrument the ROM to run from SRAM and with support for OTP emulation. This is to
enable `rom end-2-end` coverage testing.
- [ ] Enable OTP emulation for the `ROM_EXT`. Need to check if this is actually needed.
- [ ] Collect coverage metrics across test environments (i.e. `unittest`, `functest`, `end-2-end`.
- [ ] Generate aggregated coverage reports. The report should include git release information.
- [ ] Integrate coverage reports into nightly software regressions.
- [ ] Integrate coverage reports into release flows.