[libc] Move strfrom* test cases into each respective unit test. (#218474)
Almost all of the assertions are type-specific, so the helper does not
reduce duplication very much and makes it more difficult to see
coverage. `strfromf128` will be added soon and would make this worse.
`strfromf` coverage for inf/nan was missing and turns out to not work
correctly. The test case is compiled out for now, and tracked in
#217708.
Also removing the test cases for invalid format string that are
verifying specific behavior of what is UB according to the spec.
Assisted-by: Automated tooling, human reviewed.