tests: use `assertAlmostEqual` for float comparison

Floating point arithmetics often lead to unsignificant differences between
expected and computed values (this is the typical example of `0.1 + 0.2 !=
0.3`). Because of this, some tests using `assertEqual` to compare two floats
were failing. The Python method
[`assertAlmostEqual`](https://docs.python.org/2.7/library/unittest.html#unittest
.TestCase.assertAlmostEqual) can prevent these issues by comparing values up to
the seventh decimal place. To find all occurences, two RegExp were ran on all
files of the `tests/` folder: `assertEquals?(\\(.+, [0-9]+\\.[0-9]+\\))` and
`assertEquals?(\\([0-9]+\\.[0-9]+)`.

/cc @xavier_thompson @tomo
/reviewed-by @jerome
/reviewed-on !136 slapos.core!719