test/lit: Make abs() integration test slightly more interesting

I had accidentally not included this local diff in the PR that was
merged as commit cdaf554736.

Doesn't include tests for any of the interesting edge cases yet
(nans, infinities, signed zero, int.min), some of which might be
handled differently than by the host Python. In particular, the
select condition should logically use copysign(arg, 1) for the
comparison (i.e., always clear the sign bit), but currently the
chance of anyone running FP-heavy algorithms where this would
matter on the core device is close to zero [1].

[1] Sign of your choice.

artiq/test/lit/integration/abs.py

@@ -3,5 +3,5 @@
 
 assert abs(1234) == 1234
 assert abs(-1234) == 1234
-assert abs(1234.0) == 1234.0
-assert abs(-1234.0) == 1234
+assert abs(1234.5) == 1234.5
+assert abs(-1234.5) == 1234.5