Add tests for recent string package fixes (#1438)
Add tests for recent string package fixes (#1438)

This adds tests that were waiting on some interdependent PRs
to merge.

Tests are for issues #1426 and #1435
CausalityLtd/ponyc,Praetonus/ponyc,CausalityLtd/ponyc,ponylang/ponyc,cquinn/ponyc,jemc/ponyc,sgebbie/ponyc,cquinn/ponyc,Perelandric/ponyc,sgebbie/ponyc,Praetonus/ponyc,Perelandric/ponyc,sgebbie/ponyc,ponylang/ponyc,Perelandric/ponyc,jupvfranco/ponyc,jupvfranco/ponyc,kulibali/ponyc,kulibali/ponyc,cquinn/ponyc,jupvfranco/ponyc,mkfifo/ponyc,dipinhora/ponyc,Theodus/ponyc,Theodus/ponyc,ponylang/ponyc,boemmels/ponyc,cquinn/ponyc,Praetonus/ponyc,jupvfranco/ponyc,CausalityLtd/ponyc,sgebbie/ponyc,mkfifo/ponyc,Theodus/ponyc,kulibali/ponyc,Theodus/ponyc,boemmels/ponyc,sgebbie/ponyc,mkfifo/ponyc,Perelandric/ponyc,Perelandric/ponyc,mkfifo/ponyc,boemmels/ponyc,dipinhora/ponyc,jemc/ponyc,Praetonus/ponyc,boemmels/ponyc,kulibali/ponyc,Theodus/ponyc,mkfifo/ponyc,jupvfranco/ponyc,jemc/ponyc,boemmels/ponyc,dipinhora/ponyc