Add pytest-xdist dependency and -n auto option (#637)

Author: sfc-gh-stan

setup.py

@@ -263,6 +263,7 @@ def _get_arrow_lib_as_linker_input(self):
             'pytest-cov',
             'pytest-rerunfailures',
             'pytest-timeout',
+            'pytest-xdist',
             'coverage',
             'pexpect',
             'mock',

tox.ini

@@ -46,6 +46,7 @@ setenv =
     unit-integ: SNOWFLAKE_TEST_TYPE = (unit or integ)
     !unit-!integ: SNOWFLAKE_TEST_TYPE = (unit or integ)
     unit: SNOWFLAKE_TEST_TYPE = unit
+    unit: SNOWFLAKE_PYTEST_OPTS = {env:SNOWFLAKE_PYTEST_OPTS:} -n auto
     integ: SNOWFLAKE_TEST_TYPE = integ
     # Add common parts into pytest command
     SNOWFLAKE_PYTEST_COV_LOCATION = {env:JUNIT_REPORT_DIR:{toxworkdir}}/junit.{envname}-{env:cloud_provider:dev}.xml
@@ -87,6 +88,7 @@ deps =
     pytest-cov
     pytest-rerunfailures
     pytest-timeout
+    pytest-xdist
     mock
 skip_install = True
 setenv = {[testenv]setenv}