tests/fp: split and audit the conversion tests

tests/fp: split and audit the conversion tests

Split the float conversion tests into separate groups and audit the
tests to check what is still broken. I was able to enable a bunch of
tests that had been missed before:

  all the float to float conversions
  ui32_to_extF80
  ui64_to_extF80
  extF80_to_ui32
  extF80_to_ui32_r_minMag
  extF80_to_ui64
  extF80_to_ui64_r_minMag

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200520140541.30256-7-alex.bennee@linaro.org>