[Backend Tester] Add test flow CLI arg (#13360)

Add a CLI arg to filter by test flow. We have a backend filter, but this
allows for selecting only a quantized flow, for example.

Verified with the following. Only the (unquantized) CoreML flow was run.
```
python -m executorch.backends.test.suite.runner operators --filter test_add --flow coreml
```

Author: GregoryComer

backends/test/suite/discovery.py

@@ -27,6 +27,9 @@ class TestFilter:
     backends: set[str] | None
     """ The set of backends to include. If None, all backends are included. """
 
+    flows: set[str] | None
+    """ The set of test flows to include. If None, all backends are included. """
+
     name_regex: Pattern[str] | None
     """ A regular expression to filter test names. If None, all tests are included. """
 
@@ -86,6 +89,9 @@ def _is_test_enabled(test_case: unittest.TestCase, test_filter: TestFilter) -> b
     if test_filter.backends is not None and flow.backend not in test_filter.backends:
         return False
 
+    if test_filter.flows is not None and flow.name not in test_filter.flows:
+        return False
+
     if test_filter.name_regex is not None and not test_filter.name_regex.search(
         test_case.id()
     ):

backends/test/suite/runner.py

@@ -251,6 +251,7 @@ def parse_args():
     parser.add_argument(
         "-b", "--backend", nargs="*", help="The backend or backends to test."
     )
+    parser.add_argument("-l", "--flow", nargs="*", help="The flow or flows to test.")
     parser.add_argument(
         "-f", "--filter", nargs="?", help="A regular expression filter for test names."
     )
@@ -273,6 +274,7 @@ def parse_args():
 def build_test_filter(args: argparse.Namespace) -> TestFilter:
     return TestFilter(
         backends=set(args.backend) if args.backend is not None else None,
+        flows=set(args.flow) if args.flow is not None else None,
         name_regex=re.compile(args.filter) if args.filter is not None else None,
     )