sys/fmt: Missing tests for fmt_float, fmt_lpad

[jnohlgard]

See title.
Also, fmt_float has a bug where it will skip over a byte before the decimal point in the middle of the output string if the number is negative because of a mistake in the output pointer advancement.

[kaspar030]

+1, but fmt_lpad() does have some unittests?

[jnohlgard]

Maybe I missed it while I was scanning the source. But fmt_float needs some tests at least

[stale]

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[maribu]

lpad is covered in the unit tests, tests/fmt_print tests print_float() which indirectly tests fmt_float().