tests/tcg/x86_64/fma: add test for exact-denormal output

Add some fma test cases that check for correct handling of FTZ and
for the flag that indicates that the input denormal was consumed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250519145114.2786534-5-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>