assert: fix failure message hidden when a diff line exceeds 64KiB

[malikov73]

Summary

Bug fix. Fixes #746.

indentMessageLines uses bufio.Scanner, which errors out on lines longer than bufio.MaxScanTokenSize (64KiB). When that happens, the entire labeled content is replaced with cannot display message: bufio.Scanner: token too long, so the user sees neither the diff nor the values — only an empty Error: section.

truncatingFormat protects the expected:/actual: representations, but diff() output is not truncated, so this still reproduces today, e.g.:

longLine := strings.Repeat("a", bufio.MaxScanTokenSize)
assert.Equal(t, longLine+"expected", longLine+"actual")

prints

    Error Trace:	example_test.go:12
    Error:      	cannot display message: bufio.Scanner: token too long
    Test:       	TestExample

The same happens for structs with a long string field (spew prints strings on one line) and for a long single-line user message in msgAndArgs.

Changes

• indentMessageLines splits the message with strings.Split instead of bufio.Scanner, so there is no line-length limit and the cannot display message failure mode is gone. bufio.ScanLines semantics are preserved: the empty element after a trailing final "\n" is ignored and "\r\n" is treated as a line ending (a trailing "\r" is stripped from each line).
• Regression test TestEqualLongDiffLines (fails on master with cannot display message: bufio.Scanner: token too long, passes with the fix).

No API changes; output is byte-for-byte identical for messages that the scanner could already handle.