mbedtls: fix test binaries not being stripped

[nalajcie]

Description

Test binaries were unnecessarily large, with DEBUG=1 they took 140MB on rootfs in total!

Motivation and Context

JIRA: CI-494

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

7 723 tests  ±0   7 008 ✅ ±0   40m 5s ⏱️ + 2m 6s
  443 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 043f335. ± Comparison against base commit 707ec1d.