Be able to run coverage on all toxenvs.

Shifts coverage into an optional CLI wrapper that's dropped in
only when enabled via an environment variable.

This means any toxenv can now be wrapped into a coverage run
by running e.g. `tox -e py38-format-coverage` instead of
`tox -e py38-format-tests`.

Credits to @nedbat and @The-Compiler.

Author: Julian

.github/workflows/coverage.yml

@@ -20,6 +20,6 @@ jobs:
     - name: Install tox
       run: python -m pip install tox
     - name: Collect & Upload Coverage
-      run: python -m tox -e codecov
+      run: python -m tox -e pypy3-format-coverage
       env:
         CODECOV_TOKEN: 2b38dae1-41c4-4435-a29d-79a1299e5617

tox.ini

@@ -10,18 +10,29 @@ skipsdist = True
 
 [testenv]
 changedir = {envtmpdir}
+passenv = CODECOV* CI
 setenv =
     JSON_SCHEMA_TEST_SUITE = {toxinidir}/json
+
+    coverage,codecov: MAYBE_COVERAGE = coverage run -m
+    coverage,codecov: COVERAGE_RCFILE={toxinidir}/.coveragerc
+    coverage,codecov: COVERAGE_DEBUG_FILE={envtmpdir}/coverage-debug
+    coverage,codecov: COVERAGE_FILE={envtmpdir}/coverage-data
 whitelist_externals =
     mkdir
 commands =
     noextra: {envpython} -m pip install {toxinidir}
     format: {envpython} -m pip install '{toxinidir}[format]'
     format_nongpl: {envpython} -m pip install '{toxinidir}[format_nongpl]'
 
-    tests: {envpython} -m twisted.trial {posargs:jsonschema}
+    tests,coverage: {envpython} -m {env:MAYBE_COVERAGE:} twisted.trial {posargs:jsonschema}
     tests: {envpython} -m doctest {toxinidir}/README.rst
 
+    coverage: {envpython} -m coverage report --show-missing
+    coverage: {envpython} -m coverage html --directory={envtmpdir}/htmlcov
+    codecov: {envpython} -m coverage xml -o {envtmpdir}/coverage.xml
+    codecov: codecov --required --disable gcov --file {envtmpdir}/coverage.xml
+
     perf: mkdir {envtmpdir}/benchmarks/
     perf: {envpython} {toxinidir}/jsonschema/benchmarks/issue232.py --inherit-environ JSON_SCHEMA_TEST_SUITE --output {envtmpdir}/benchmarks/issue232.json
     perf: {envpython} {toxinidir}/jsonschema/benchmarks/json_schema_test_suite.py --inherit-environ JSON_SCHEMA_TEST_SUITE --output {envtmpdir}/benchmarks/json_schema_test_suite.json
@@ -69,17 +80,6 @@ deps =
 commands =
     {envpython} -m flake8 {posargs} {toxinidir}/jsonschema {toxinidir}/docs
 
-[testenv:coverage]
-setenv =
-    {[testenv]setenv}
-    COVERAGE_DEBUG_FILE={envtmpdir}/coverage-debug
-    COVERAGE_FILE={envtmpdir}/coverage-data
-commands =
-    {envpython} -m pip install '{toxinidir}[format]'
-    {envpython} -m coverage run --rcfile={toxinidir}/.coveragerc -m twisted.trial jsonschema
-    {envpython} -m coverage report --rcfile={toxinidir}/.coveragerc --show-missing
-    {envpython} -m coverage html --directory={envtmpdir}/htmlcov --rcfile={toxinidir}/.coveragerc {posargs}
-
 [testenv:docs-html]
 basepython = pypy3
 commands = {envpython} -m sphinx -b html {toxinidir}/docs/ {envtmpdir}/build {posargs:-a -n -q -T -W}
@@ -115,12 +115,3 @@ deps =
     doc8
     pygments
     pygments-github-lexers
-
-[testenv:codecov]
-passenv = CODECOV* CI
-setenv = {[testenv:coverage]setenv}
-commands =
-    {envpython} -m pip install '{toxinidir}[format]'
-    {envpython} -m coverage run --rcfile={toxinidir}/.coveragerc -m twisted.trial jsonschema
-    {envpython} -m coverage xml -o {envtmpdir}/coverage.xml
-    codecov --required --disable gcov --file {envtmpdir}/coverage.xml