Format comment before parameter default correctly (#7870)
**Summary** Handle comment before the default values of function
parameters correctly by inserting a line break instead of space after
the equals sign where required.
```python
def f(
a = # parameter trailing comment; needs line break
1,
b =
# default leading comment; needs line break
2,
c = ( # the default leading can only be end-of-line with parentheses; no line break
3
),
d = (
# own line leading comment with parentheses; no line break
4
)
)
```
Fixes #7603
**Test Plan** Added the different cases and one more complex case as
fixtures.
