The multiline pattern of awslogs shim logger cannot log the last matched line in e2e tests

[ningziwen]

During my test against awslogs shim logger, the last matched line cannot be logged when multiline pattern is set.

For example, when the multiline pattern is ^INFO, and the log printed by container is

INFO a
INFO b
INFO c

Only INFO a and INFO b can be passed to awslogs.

This may be caused by test set up or the bug of the feature itself.

Need more investigation. Some potential directions to investigate:

• Try to reproduce with Nerdctl by specifying the shim logger binary path.
• The shim logger depends on the log drivers in Moby. So we can try to reproduce by Docker with its native awslogs driver.
• The logic here seems processing event first with buffer and then append to the buffer. So it is possible that the last line's content is attached to the buffer finally but does not have chance to be processed.
• The current awslogs test infra uses AWS local stack , so may have feature gaps.