test: add test to check if a format string is plain

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (a583ffa) 99.40% compared to head (c59f50f) 99.42%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #65 +/- ## ========================================== + Coverage 99.40% 99.42% +0.01% ========================================== Files 56 56 Lines 5866 5877 +11 ========================================== + Hits 5831 5843 +12 + Misses 35 34 -1 ``` | [Files](https://app.codecov.io/gh/Viatorus/emio/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Toni+Neubert) | Coverage Δ | | |---|---|---| | [include/emio/buffer.hpp](https://app.codecov.io/gh/Viatorus/emio/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Toni+Neubert#diff-aW5jbHVkZS9lbWlvL2J1ZmZlci5ocHA=) | `99.35% <100.00%> (ø)` | | | [test/unit\_test/detail/test\_dragon.cpp](https://app.codecov.io/gh/Viatorus/emio/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Toni+Neubert#diff-dGVzdC91bml0X3Rlc3QvZGV0YWlsL3Rlc3RfZHJhZ29uLmNwcA==) | `100.00% <ø> (ø)` | | | [test/unit\_test/test\_buffer.cpp](https://app.codecov.io/gh/Viatorus/emio/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Toni+Neubert#diff-dGVzdC91bml0X3Rlc3QvdGVzdF9idWZmZXIuY3Bw) | `100.00% <100.00%> (ø)` | | | [test/unit\_test/test\_format\_string.cpp](https://app.codecov.io/gh/Viatorus/emio/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Toni+Neubert#diff-dGVzdC91bml0X3Rlc3QvdGVzdF9mb3JtYXRfc3RyaW5nLmNwcA==) | `100.00% <100.00%> (ø)` | | | [test/unit\_test/test\_writer.cpp](https://app.codecov.io/gh/Viatorus/emio/pull/65?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Toni+Neubert#diff-dGVzdC91bml0X3Rlc3QvdGVzdF93cml0ZXIuY3Bw) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/Viatorus/emio/pull/65/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Toni+Neubert)

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[Viatorus]

ICE with clang-15...