Fix rounding bug when formatting floats with minimal_printf

[sytsereitsma]

Summary of changes

Fixed error in floating point rounding in minimal_printf implementation:

• The decimals were 'rounded' after the integer part was printed. For example 6.99 with "%.1f" would be formatted as "6.10" instead of "7.0"
• Rounding of numbers with 0 decimals only worked for positive numbers. For example -6.99 with "%.0f" would be formatted as "-5" instead of "-7".

Added unit tests for floating point rounding and string padding (aka prepending) in minimal_printf.

Impact of changes

Migration actions required

Documentation

None

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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)

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Test results

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

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Reviewers

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[ciarmcom]

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

[0xc0170]

Please amend the commit message for 6d58014 (it should explain how is it fixing the bug)

[0xc0170]

@sytsereitsma Thank you for contributing to Mbed OS. I've noticed you added also unit tests, great ! I left a git request above, please review

Pull request has been modified.

[0xc0170]

I enabled to run basic checks (I expect style check to fail, few offenders there, please review once completed).

[mbed-ci]

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