Truncate very long objects in test failure messages

[brackendawson]

Summary

Truncate object representations in test failure output messages that would cause the message to be too long to scan.

This is a different approach than that in #1559. I feel this is more appropriate because is it safer. The other approach has the potential to allocate excessive memory and to print excessive output to the console.

Changes

• Make the message line indenter show an appropriate error if it fails to scan a line from the message.
• Use the existing truncatingFormat function to truncate potentially long objects in assertions which format expected/actual values in full.
• Parameterize the format string used by truncatingFormat.
• Reduce the limit used by truncatingFormat by almost half to support failure messages showing both long expected and long actual values on the same line.
• Change the format of NotSubset from %q to %#v to match that of Subset.

Motivation

So that people don't see no output in the failure message when making assertions (such as Len) against very long objects.

Related issues

Closes #1525