Fix coverage in sytest and use plugins for buildkite (#5922)

tox.ini

@@ -7,6 +7,7 @@ deps =
     python-subunit
     junitxml
     coverage
+    coverage-enable-subprocess
     parameterized
 
     # cyptography 2.2 requires setuptools >= 18.5
@@ -43,13 +44,13 @@ whitelist_externals =
 setenv =
     {[base]setenv}
     postgres: SYNAPSE_POSTGRES = 1
+    TOP={toxinidir}
 
 passenv = *
 
 commands =
     /usr/bin/find "{toxinidir}" -name '*.pyc' -delete
     # Add this so that coverage will run on subprocesses
-    sh -c 'echo "import coverage; coverage.process_startup()" > {envsitepackagesdir}/../sitecustomize.py'
     {envbindir}/coverage run "{envbindir}/trial" {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
 
 # As of twisted 16.4, trial tries to import the tests as a package (previously
@@ -75,8 +76,6 @@ commands =
 # )
 usedevelop=true
 
-
-
 # A test suite for the oldest supported versions of Python libraries, to catch
 # any uses of APIs not available in them.
 [testenv:py35-old]
@@ -88,6 +87,7 @@ deps =
     mock
     lxml
     coverage
+    coverage-enable-subprocess
 
 commands =
     /usr/bin/find "{toxinidir}" -name '*.pyc' -delete
@@ -96,15 +96,11 @@ commands =
     # OpenSSL 1.1 compiled cryptography (as older ones don't compile on Travis).
     /bin/sh -c 'python -m synapse.python_dependencies | sed -e "s/>=/==/g" -e "s/psycopg2==2.6//" -e "s/pyopenssl==16.0.0/pyopenssl==17.0.0/" | xargs -d"\n" pip install'
 
-    # Add this so that coverage will run on subprocesses
-    /bin/sh -c 'echo "import coverage; coverage.process_startup()" > {envsitepackagesdir}/../sitecustomize.py'
-
     # Install Synapse itself. This won't update any libraries.
     pip install -e .
 
     {envbindir}/coverage run "{envbindir}/trial"  {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
 
-
 [testenv:packaging]
 skip_install=True
 deps =
@@ -137,15 +133,15 @@ basepython = python3.6
 [testenv:check-sampleconfig]
 commands = {toxinidir}/scripts-dev/generate_sample_config --check
 
-[testenv:codecov]
+[testenv:combine]
 skip_install = True
 deps =
     coverage
-    codecov
-commands =
+whitelist_externals =
+    bash
+commands=
     coverage combine
-    coverage xml
-    codecov -X gcov
+    coverage report
 
 [testenv:mypy]
 basepython = python3.5
@@ -155,4 +151,4 @@ deps =
 extras = all
 commands = mypy --ignore-missing-imports \
             synapse/logging/_structured.py \
-            synapse/logging/_terse_json.py
+            synapse/logging/_terse_json.py