Add failing test for poetry.lock

Check that pinned requirements are honored when poetry.lock is present.
Request an outdated version of marshmallow in poetry.lock (3.0.0). Check
that the outdated version is installed, rather than a newer version
also satisfying the version constraint in pyproject.toml (^3.0.0).

Author: cjolowicz

test/fixtures/poetry-lock/poetry.lock

@@ -6,7 +6,7 @@ authors = ["Your Name <[email protected]>"]
 
 [tool.poetry.dependencies]
 python = "^3.6"
-attrs = "^19.1.0"
+marshmallow = "^3.0.0"
 
 [tool.poetry.dev-dependencies]
 

test/fixtures/poetry-lock/pyproject.toml

@@ -82,6 +82,7 @@ testPoetry() {
 
 testPoetryLock() {
     compile "poetry-lock"
+    assertCaptured "marshmallow==3.0.0"
     assertCapturedSuccess
 }