Standard test suite for pascals-triangle (#362)
Standard test suite for pascals-triangle (#362)

In surveying the current implementations, I found 2 main approaches:

- Test 1, 2, 3, 4, 5 and 20 without any boundary checking
  - [Ruby](https://github.com/exercism/xruby/blob/e50b1cbeabc3a82c1ed7800da112b0984dbbf01c/exercises/pascals-triangle/pascals_triangle_test.rb)
    is a good example
- Test 4 & 6 rows, and test boundaries
  - [Kotlin](https://github.com/exercism/xkotlin/blob/02ecc5a70719f0b0e741992f33879f880f7c407f/exercises/pascals-triangle/src/test/kotlin/PascalsTriangleTest.kt)
    is a good example

Also, some languages exposed specific functions for `rows` and
`last_row` while some relied on built-in functions for this behavior.

In this approach I'm trying to meld what I see as the best of these
worlds.

Rows function
____

I think these functions should be implemented by the students, so I'm
highlighting them as functions to test

We decided that `last_row` functions were not necessary, since they
would mostly only alias existing language functions (`pop`, `last`,
etc.)

Fewer Tests
----

Once a student has gotten 4 rows to work the probably have the
algorithm down. No real reason to test larger numbers that I can see.

Test Boundaries
----

An upper-limit test seemed unnecessary and very language-dependent. But
catching Nil and negative inputs seemed like useful things to test.
Vankog/problem-specifications,ErikSchierboom/x-common,Vankog/problem-specifications,ErikSchierboom/x-common,kgengler/x-common,jmluy/x-common,exercism/x-common,rpottsoh/x-common,petertseng/x-common,jmluy/x-common,petertseng/x-common,kgengler/x-common,exercism/x-common,jmluy/x-common,rpottsoh/x-common