Allow negative testing in pipeline tests

[andrewkroh]

It is useful to be able to create negative test cases for Ingest Node pipelines in order to check for proper error handling when unexpected data is received. For example when an invalid IP address is received in a log message part of the error handling is to remove the invalid value to prevent mapping failures at ingest time and append an error.message indicating that the data was invalid.

So in the pipeline test config we need a way to ignore failures caused by the presence of error.message and continue to assert that golden data matches the output.

[andrewkroh]

Relates: #327 (in that we probably need to accept error.message as an array)

[jsoriano]

Initial support added in #1333.

[chrisberkhout]

#1333 allows system tests to check for too many documents, by asserting an exact hit count rather than a minimum hit count.

The testing of error handling logic in pipeline tests, as described above by @andrewkroh, is not yet possible.

An example use case for this type of test can be seen here: elastic/integrations#8875 (comment).