Pretty printing: add parentheses around Relationals

Fixes #7117.
LaTeX printing: minor simplification

We don't need a result variable "res": simply return False if none
of the various conditions became true.
LaTeX printing: whitespace fixes in tests

Minor two-lines-between-functions fixes while I was scanning
through these tests.
LaTeX printing: add parentheses around Relationals

Fixes #7117.  Relationals need parentheses when inside a Mul, an easy
fix to `_needs_mul_brackets`.  They also need parentheses inside an
Add, apparently the first thing to do so, so a `_needs_add_brackets`
function was added.  Then `_print_Add` was modified to check this.

Also adds tests.
Merge pull request #8566 from cbm755/fix7117

Pretty printing: add parentheses around Relationals