The existing test for `math.e` and `math.pi` constants can fail on certain
targets if the functions `math.exp()` and/or `math.cos()` are not accurate
enough (eg out by an LSB of float precision). For example this test
currently fails on PYBD_SF6 which uses double precision floats (and that's
due to the `lib/libm_dbl/exp.c` implementation not being exact).
This commit changes this constant test so that it tests the actual constant
value, not the evaluation of `exp()` and `cos()` functions.
Signed-off-by: Damien George <damien@micropython.org>
