Exiting with zero exit code in case of errors in bad style, but we do
not want to fail CI test run if coveralls.io is down. For other error
(e. g. internal scrip errors) exit code still be non zero.

Sending functional test log (about 5Mb) in a email attachmend was temporary
solution.
Upload them to https://ci.rspamd.com/testlogs/ instead.
Link to log/report will be printed in build log.

* Add utility to prepare C coverage for upload to coveralls.io

It turned out that it is more easy to write own script then debug
and fix coveralls utility (https://github.com/eddyxu/cpp-coveralls).

gcov-coveralls.py can be used as a replacement for coveralls.

* Save coverage data from .gcda files only once

Coverage data in .gcda files is merged after each binary invocation,
so we can run all test and then gather coverage data.

If we dump them two times execution counts will be more then they be.

* Switch from coveralls (cpp-coveralls) to own script

Problem with coveralls was, that coverage for source files outside build
directory was not added to the report.

* Add tool to dump info from json for coveralls.io

* Add debug

* Fix: don't die if there is no service_job_id in json

* Debug

* Fix dump_coveralls.py

* Rename to gcov_coveralls.py (s/-/_/)

For most files in this repo '_' is used as separator.

* Don't add source code to coveralls JSON

According to https://docs.coveralls.io/api-introduction
Coverals don't need source code, only MD5 digest to tracks changes.

Anyway source code is already added by luacov-coveralls and source_digest
is added by cpp-coveralls and gcov_coveralls.py
Both options seems to work for now.

* Provide path to source directory to merge_coveralls.py

merge_coveralls.py has code to filter files and remove prefixes.
When --root points to source directory merge_coveralls.py can strip
prefix from absolute path in JSONs generated by luacov-coveralls.

* Style

Don't add parameters with default values.

* Make --output optional

It useful mainly for debugging. We can send report without saving it.

* Log CI_COMMIT_AUTHOR env var

It is not clear from drone.io source how CI_COMMIT_AUTHOR variable is
set. Log it to see what it means.

* Move merge_coveralls.py to test/tools

This script is used not only for funcional test coverage, but for
rspamd-test coverage too.

* Remove debug

* Style

Use more compact formatting.

* Write comment about parallel tests running [SKIP CI]

Document why running tests in parallel may be bad idea (but still do so).

* Fix typo [SKIP CI]