Make test_symbols.py compare files line-by-line

We currently feed full files to Python's unified_diff.
It's not quite what we want though -
line-by-line comparison makes more sense
(we want to be able to identify missing/unnecessary lines)
and is also easier to parse for humans.
This patch makes sure that we compare one line at a time.

This change pretties up the output formatting in the script.
Output before:

```
!DEF:/m/s/xINTENT(IN)(Implicit)ObjectEntityREAL(4)
!DEF:/m/s/yINTENT(INOUT)(Implicit)ObjectEntityREAL(4)
-!-D-E-F-:-f-o-o-b-a-r-
puresubroutines(x,y)bind(c)
!REF:/m/s/x
intent(in)::x
```
Proposed output after:

```
!DEF:/m/s/xINTENT(IN)(Implicit)ObjectEntityREAL(4)
!DEF:/m/s/yINTENT(INOUT)(Implicit)ObjectEntityREAL(4)
-!DEF:foobar
puresubroutines(x,y)bind(c)
!REF:/m/s/x
intent(in)::x
```

Reviewed By: Meinersbur, awarzynski

Differential Revision: https://reviews.llvm.org/D107954