libc: re-enable tests for strtoll, strtoull

[jmaksymowicz]

Also add test for strtol, etc. to test for case where "0x" prefix is not followed by a hex digit.

Description

Motivation and Context

Re-enable tests after fixes in phoenix-rtos/libphoenix#381

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

7 958 tests  +25   7 256 ✅ +41   40m 17s ⏱️ +4s
  461 suites +16     702 💤  - 16 
    1 files   ± 0       0 ❌ ± 0 

Results for commit e926de2. ± Comparison against base commit c510830.

♻️ This comment has been updated with latest results.