Improve fixed format testing and fix 0 insertion bugs

[mborland]

Closes: #205

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.18605% with 5 lines in your changes missing coverage. Please review.

Project coverage is 94.52%. Comparing base (8e46c88) to head (6391804).

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/charconv/pull/206?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #206 +/- ## =========================================== + Coverage 94.01% 94.52% +0.51% =========================================== Files 66 66 Lines 8439 8459 +20 =========================================== + Hits 7934 7996 +62 + Misses 505 463 -42 ``` | [Files](https://app.codecov.io/gh/boostorg/charconv/pull/206?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [...lude/boost/charconv/detail/dragonbox/dragonbox.hpp](https://app.codecov.io/gh/boostorg/charconv/pull/206?src=pr&el=tree&filepath=include%2Fboost%2Fcharconv%2Fdetail%2Fdragonbox%2Fdragonbox.hpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9jaGFyY29udi9kZXRhaWwvZHJhZ29uYm94L2RyYWdvbmJveC5ocHA=) | `97.67% <ø> (+0.30%)` | :arrow_up: | | [test/roundtrip.cpp](https://app.codecov.io/gh/boostorg/charconv/pull/206?src=pr&el=tree&filepath=test%2Froundtrip.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC9yb3VuZHRyaXAuY3Bw) | `98.34% <100.00%> (+0.05%)` | :arrow_up: | | [test/to\_chars\_float.cpp](https://app.codecov.io/gh/boostorg/charconv/pull/206?src=pr&el=tree&filepath=test%2Fto_chars_float.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90b19jaGFyc19mbG9hdC5jcHA=) | `99.66% <100.00%> (+<0.01%)` | :arrow_up: | | [test/to\_chars\_float\_STL\_comp.cpp](https://app.codecov.io/gh/boostorg/charconv/pull/206?src=pr&el=tree&filepath=test%2Fto_chars_float_STL_comp.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90b19jaGFyc19mbG9hdF9TVExfY29tcC5jcHA=) | `100.00% <100.00%> (ø)` | | | [test/to\_chars\_sprintf.cpp](https://app.codecov.io/gh/boostorg/charconv/pull/206?src=pr&el=tree&filepath=test%2Fto_chars_sprintf.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90b19jaGFyc19zcHJpbnRmLmNwcA==) | `96.44% <ø> (-0.07%)` | :arrow_down: | | [src/to\_chars\_float\_impl.hpp](https://app.codecov.io/gh/boostorg/charconv/pull/206?src=pr&el=tree&filepath=src%2Fto_chars_float_impl.hpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-c3JjL3RvX2NoYXJzX2Zsb2F0X2ltcGwuaHBw) | `85.71% <79.16%> (+3.80%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/boostorg/charconv/pull/206/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/charconv/pull/206?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/boostorg/charconv/pull/206?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [8e46c88...6391804](https://app.codecov.io/gh/boostorg/charconv/pull/206?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg).