… messages

As pointed out in stretchr#1525, when the assertion message is too long, it gets
completely truncated in the final output. This is because,
`bufio.Scanner.Scan()` has a default `MaxScanTokenSize` set to `65536`
characters (64 * 1024). The `Scan()` function return false whenever the
line being scanned exceeds that max limit. This leads to the final
assertion message being truncated.

This commit fixes that by manually setting the internal scan buffer size
to `len(message) + 1` to make sure that above scenario never occurs.

Fixes stretchr#1525

Instead of using an arbitrary value like 20000, we can just use the
value defined by bufio package (MaxScanTokenSize).

Also, fix the test cases for this function. This commit generates the
input based on parameters like bytes per line and number of lines. The
assertion is made against the pattern of the output rather than the
exact output.