Fix the way tests run

Currently `torchtext` runs tests by `pytest test`. This is fine for development but one cannot run unit test on installed package in this manner because running test from the root adds the current directory as module search path and this will shadow the installed package. Similarly if you do `(cd test && pytest .)`, this does not work either because `pytest` will figure out where the test module root starts and add the repository root to python module search path.

When running test on CI, it is more desirable to install the package and run tests because it can catch a bug related to packaging. Such as `zip_safe=False` issue. (this cannot be caught with the current configuration)

To resolve this issue, this PR
1. introduces `test/torchtext_unittest` directory and moves all the test scripts and assets there.
2. changes `python setup.py develop` to `python setup.py install` in CI job
3. run test with `(cd test && pytest torchtext_unittest)` so that repository root is not in Python module search path.
4. Change relative import (`from ..common import ...`) in test module to absolute import (`from torchtext_unittest.common import ...`)
5. deletes `pytest.ini`, which hard-coded test path.

Author: mthrok

.circleci/unittest/linux/scripts/install.sh

@@ -15,7 +15,7 @@ conda install -y -c "pytorch-${UPLOAD_CHANNEL}" ${CONDA_CHANNEL_FLAGS} pytorch c
 
 printf "* Installing torchtext\n"
 git submodule update --init --recursive
-python setup.py develop
+python setup.py install
 
 printf "* Installing parameterized\n"
 pip install parameterized

.circleci/unittest/linux/scripts/run_test.sh

@@ -6,4 +6,9 @@ eval "$(./conda/bin/conda shell.bash hook)"
 conda activate ./env
 
 python -m torch.utils.collect_env
-pytest --cov=torchtext --junitxml=test-results/junit.xml -v --durations 20 test
+
+# Do not run pytest from the root directory of the repository.
+# The checked out code will be added to the Python module search path and
+# will shadow the development/instaled package.
+cd test
+pytest --cov=torchtext --junitxml=test-results/junit.xml -v --durations 20 torchtext_unittest

.circleci/unittest/windows/scripts/install.sh

@@ -20,7 +20,7 @@ conda install -y -c "pytorch-${UPLOAD_CHANNEL}" ${CONDA_CHANNEL_FLAGS} pytorch c
 
 printf "* Installing torchtext\n"
 git submodule update --init --recursive
-"$root_dir/packaging/vc_env_helper.bat" python setup.py develop
+"$root_dir/packaging/vc_env_helper.bat" python setup.py install
 
 printf "* Installing parameterized\n"
 pip install parameterized

.circleci/unittest/windows/scripts/run_test.sh

@@ -6,4 +6,9 @@ eval "$(./conda/Scripts/conda.exe 'shell.bash' 'hook')"
 conda activate ./env
 
 python -m torch.utils.collect_env
-pytest --cov=torchtext --junitxml=test-results/junit.xml -v --durations 20 test
+
+# Do not run pytest from the root directory of the repository.
+# The checked out code will be added to the Python module search path and
+# will shadow the development/instaled package.
+cd test
+pytest --cov=torchtext --junitxml=test-results/junit.xml -v --durations 20 torchtext_unittest