pov 1.1.1.3: simplify "Can find path from nodes other than x"

[petertseng]

This was the main change in exercism/problem-specifications#704

exercism/problem-specifications#710 is an inconsequential
change that does not affect our tests.

Two other changes in 1.1.0 that aren't exactly followed:

• As a part of conforming to the canonical schema, trees were embedded
  into the test data, but we have no need to do it - let's reduce
  duplication, and besides we are safe from accidental mutation.
• The "cannot trace" cases were changed to use simpler trees than
  cousins, but we would have had to introduce another tree to do that
  (one with kids and siblings). We'll stick with cousins.

[petertseng]

I'm honestly not sure about the decisions I made, so take a look.

[rbasso]

• As a part of conforming to the canonical schema, trees were embedded into the test data, but we have no need to do it - let's reduce duplication, and besides we are safe from accidental mutation.
• The "cannot trace" cases were changed to use simpler trees than cousins, but we would have had to introduce another tree to do that (one with kids and siblings). We'll stick with cousins.

Considering that we are not algorithmically generating the test suites, It makes sense to reuse data structures.