Fix coverage report.

[belimawr]

Currently the coverage reports are not working.
To run unit tests with coverage report for Filebeat:

cd filebeat
TEST_COVERAGE=true mage GoUnitTest

You'll see the following error:

cover: can't read "github.com/elastic/beats/v7/filebeat/input/syslog/rfc3164_parser.rl": open /home/tiago/devel/beats/filebeat/input/syslog/rfc3164_parser.rl: no such file or directory
Error: failed to write HTML code coverage report: running "go tool cover -html=build/TEST-go-unit.cov -o build/TEST-go-unit.html" failed with exit code 1

The problem seems to be caused by some files in the syslog input (like filebeat/input/syslog/rfc3164_parser.go) that contain annotations to some .rl files, those annotations look like this:

//line parser/rfc3164_parser.rl:28

Those files are autogenerated.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)