Add --no-loadscope-reorder and --loadscope-reorder options (#1217)

Based on #1098.

Closes #1098

---------

Co-authored-by: Toan Vuong <toan.vuong@hyperscience.com>

src/xdist/plugin.py

@@ -127,6 +127,32 @@ def pytest_addoption(parser: pytest.Parser) -> None:
             "(default) no: Run tests inprocess, don't distribute."
         ),
     )
+    group.addoption(
+        "--loadscope-reorder",
+        dest="loadscopereorder",
+        action="store_true",
+        default=True,
+        help=(
+            "Pytest-xdist will default reorder tests by number of tests per scope "
+            "when used in conjunction with loadscope.\n"
+            "This option will enable loadscope reorder which will improve the "
+            "parallelism of the test suite.\n"
+            "However, the partial order of tests might not be retained.\n"
+        ),
+    )
+    group.addoption(
+        "--no-loadscope-reorder",
+        dest="loadscopereorder",
+        action="store_false",
+        help=(
+            "Pytest-xdist will default reorder tests by number of tests per scope "
+            "when used in conjunction with loadscope.\n"
+            "This option will disable loadscope reorder, "
+            "and the partial order of tests can be retained.\n"
+            "This is useful when pytest-xdist is used together with "
+            "other plugins that specify tests in a specific order."
+        ),
+    )
     group.addoption(
         "--tx",
         dest="tx",