[TG-7618] Fix precision parsing in string_format

[romainbrenguier]

The 4th group in the regular expression matches (\.\d+)? so the dot
should be ignored before the string can be parsed to an int.

• Each commit message has a non-empty body, explaining why the change was made.
• [na] Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• [na] My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 9c7b44a).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/111548705

[tautschnig]

Would it be possible to include a test?

[martin-cs]

Sounds believable but a "fails before but works now" test case, either system or unit, would be good.

[romainbrenguier]

I started adding a test but it will require a model for Float in java-models-library