As reported in #235 formatting the first number which doesn't fit into
int64_t anymore fails to add the thousands separators.
I.e.:
`9223372036854775807` -> `9,223,372,036,854,775,807`
`9223372036854775808` -> `9223372036854775808`
Add a test reproducing that that for all backends.
The Drone CI fails because the en_US.UTF-8 `locale_t` does not have a
grouping and thousand separator so "12,345" fails to parse.
Format instead to have the expected format as input.
When parsing a string like "123.456" to an integer the ICU backend would
first parse it greedily to a floating point value and then cast/truncate
it to an integer.
Set the flag to only parse integers when parsing to an integral number.
Care must be taken not to set that when parsing e.g. a currency or date
to an integer where the truncation is intended.
