tests/tcg: add float_convd test

This is a simple transliteration of the float_convs test but this time
working with doubles. I'm used it to test the handling of vector
registers in gdbstub but wasn't able to find a non-ugly way to
automate it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-23-alex.bennee@linaro.org>