Fix test_installed_version to work in the mypy_mypyc CI (#5955)

misc/test_installed_version.sh

@@ -7,26 +7,34 @@
 # it annoying to test an installed version of mypy. If somebody has a
 # better way please let me know.
 
+function abspath {
+    python3 -c "import os.path; print(os.path.abspath('$1'))"
+}
+
 TO_INSTALL="${1-.}"
 PYTHON="${2-python3}"
 VENV="$(mktemp -d -t mypy-test-venv.XXXXXXXXXX)"
 trap "rm -rf '$VENV'" EXIT
 
-"$PYTHON" -m venv "$VENV"
+"$PYTHON" -m virtualenv "$VENV"
 source "$VENV/bin/activate"
 
-pip install -r test-requirements.txt
+ROOT="$PWD"
+TO_INSTALL="$(abspath "$TO_INSTALL")"
+
+# Change directory so we can't pick up any of the stuff in the root.
+# We need to do this before installing things too because I was having
+# the current mypy directory getting picked up as satisfying the
+# requirement (argh!)
+cd "$VENV"
+
+pip install -r "$ROOT/test-requirements.txt"
 pip install $TO_INSTALL
 
 # pytest looks for configuration files in the parent directories of
 # where the tests live. Since we are trying to run the tests from
 # their installed location, we copy those into the venv. Ew ew ew.
-cp pytest.ini conftest.py "$VENV/"
-
-ROOT="$PWD"
-
-# Change directory so we can't pick up any of the stuff in the root
-cd "$VENV"
+cp "$ROOT/pytest.ini" "$ROOT/conftest.py" "$VENV/"
 
 # Find the directory that mypy tests were installed into
 MYPY_TEST_DIR="$(python3 -c 'import mypy.test; print(mypy.test.__path__[0])')"