Fixed string truncation for TestReport's comment

[metenn]

Django's Truncator(text).chars(n) seems to ignore all unicode combining characters, resulting in incorrect string truncations.
For example,

string = 'After\u20e1 trunc, this should be 20 characters...'
temp = Truncator(string).chars(20)
print(len(temp))

this outputs:

21

If we were to remove the unicode symbol, it would correctly output 20. Whether this is truly intended or not eludes me.

When trying to put a string truncated in such manner into a CharField, it fails to pass the MaxLengthValidator check, as it internally uses len() for its check.

Since Python 3, strings respect Unicode by default, so slices seem to work fine for truncation.

[MasloMaslane]

This change was tested on sio2.mimuw and works