tests/float/math_constants.py: Test actual e and pi constant values.

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>

Author: dpgeorge

tests/float/math_constants.py

@@ -1,11 +1,30 @@
 # Tests various constants of the math module.
+
+import sys
+
 try:
-    import math
-    from math import exp, cos
+    from array import array
+    from math import e, pi
 except ImportError:
     print("SKIP")
     raise SystemExit
 
-print(math.e == exp(1.0))
+# Hexadecimal representations of e and pi constants.
+e_truth_single = 0x402DF854
+pi_truth_single = 0x40490FDB
+e_truth_double = 0x4005BF0A8B145769
+pi_truth_double = 0x400921FB54442D18
+
+# Detect the floating-point precision of the system, to determine the exact values of
+# the constants (parsing the float from a decimal string can lead to inaccuracies).
+if float("1e300") == float("inf"):
+    # Single precision floats.
+    e_truth = array("f", e_truth_single.to_bytes(4, sys.byteorder))[0]
+    pi_truth = array("f", pi_truth_single.to_bytes(4, sys.byteorder))[0]
+else:
+    # Double precision floats.
+    e_truth = array("d", e_truth_double.to_bytes(8, sys.byteorder))[0]
+    pi_truth = array("d", pi_truth_double.to_bytes(8, sys.byteorder))[0]
 
-print(cos(math.pi))
+print("e:", e == e_truth or (e, e_truth, e - e_truth))
+print("pi:", pi == pi_truth or (pi, pi_truth, pi - pi_truth))