Fix test execution in torchtext

[Nayef211]

Description

• Introduces test/torchtext_unittest directory and moves all the test scripts and assets there (this is in line with how torchaudio organizes it's unit tests)
• changes python setup.py develop to python setup.py install in CI job
• run test with (cd test && pytest torchtext_unittest) so that repository root is not in Python module search path.
• Change relative import (from ..common import ...) in test module to absolute import (from torchtext_unittest.common import ...)
• Creates a workflow to run integration tests seperately from unittests

Additional context

• Followup to Create pytest fixture to auto delete model checkpoints within integration tests #1886
• Stale PR that attempted to do this in the past Fix the way tests are executed #1223

Followups

• Investigate test_vocab_from_raw_text_file failure on linux platforms (Investigate test_vocab_from_raw_text_file failures on Linux #1900)
• Keeping asset dir and make integration test not access internals of torchtext_unittest

Closes #1875

[joecummings]

To confirm: Failures related to test_vocab_from_raw_text_file will be resolved in a followup PR or in this PR?

[Nayef211]

To confirm: Failures related to test_vocab_from_raw_text_file will be resolved in a followup PR or in this PR?

The plan is to investigate and resolve them in a followup PR. There aren't any significant changes in this PR that should cause that test to fail.

[joecummings]

lgtm

[mthrok]

Overall it looks good.

• test/.gitignore → test/torchtext_unittest/.gitignore: This file is empty. Do we need it?

• Potential follow-up: keeping asset dir and make integration test not access internals of torchtext_unittest would make it cleaner.

[mthrok]

To confirm: Failures related to test_vocab_from_raw_text_file will be resolved in a followup PR or in this PR?

The plan is to investigate and resolve them in a followup PR. There aren't any significant changes in this PR that should cause that test to fail.

Was it failing before?

[Nayef211]

To confirm: Failures related to test_vocab_from_raw_text_file will be resolved in a followup PR or in this PR?

The plan is to investigate and resolve them in a followup PR. There aren't any significant changes in this PR that should cause that test to fail.

Was it failing before?

Yes it was failing sporadically on certain python versions on Linux machines. Disabled them for the time being in #1901

[Nayef211]

• test/.gitignore → test/torchtext_unittest/.gitignore: This file is empty. Do we need it?

Deleted the file

• Potential follow-up: keeping asset dir and make integration test not access internals of torchtext_unittest would make it cleaner.

Added it as a followup item in PR description