To clarify, `parse_f64` will indeed take `infi` to mean `+Inf` and
return that as the value, but it will not return `ok = true`. It treats
it as `inf` followed by any other trailing character.
`parse_f64_prefix` is the lenient one which will return true so long as
it finds some meaningful value.
The full "infinity" strings were expected to be partial consumes, but
this is not the case. That has been fixed and the relevant extra tests
from that file have been added to this test suite.
Fixes#2670
