Add End-to-end test bed

[tigrannajaryan]

Problem

We need a way to run E2E tests with real agent and a load generator. Particularly
important class of E2E tests are performance tests in controlled environment.

Solution

The test bed allows to easily set up a test that requires running the agent
and a load generator, measure and define resource consumption expectations
for the agent, fail tests automatically when expectations are exceeded.

Each test case requires a agent configuration file and (optionally) load
generator spec file. Test cases are defined as regular Go tests, see examples
in perf_test.go.

To run E2E tests go to testbed directory and run "make runtests" or run "make perf-tests"
from top-level directory.

Note that E2E tests are skipped when running regular "make test" target. The reason
is that because E2E tests take a lot of time and require non-concurrent execution
while other tests are typical run with -race flag.

See also design doc here:
https://docs.google.com/document/d/1omU06mBYGY0slT1yojttn9BCyp18pHaRjkkYZrY8H4Q/edit#

TODO

We plan to add the following functionality later:

• Ability for load generator to record what it generated and then use that
  information to validate data received by mock backend from the agent.

• Enforce resource consumption limits on the agent to allow testing scenarios under
  specific available resource situations (as opposed to measuring and failing tests
  which is already implemented and which is useful in different scenarios).

[codecov-io]

Codecov Report

Merging #54 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #54   +/-   ##
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  Coverage   69.67%   69.67%           
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  Files          86       86           
  Lines        5606     5606           
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  Hits         3906     3906           
  Misses       1481     1481           
  Partials      219      219

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[owais]

LGTM

[pjanotti]

LGTM. I already used it a few times and works well on linux.

[pjanotti]

nit: headers, some other files too (I remember someone asking if that was really necessary but I don't recall seeing an answer to that).

[pjanotti]

While we shouldn't run the testbed every time it should be built together with the repo.

[pjanotti]

Fine to do it later.