test-suites-dir: Allow multiple dirs with OS path separator

Allowing multiple directories allows the creation of private test
suites. Implemented using OS path separator (`os.pathsep`). For
instance, on Linux:

```
python3 ./fluster.py -tsd "/opt/fluster_tsd:/opt/priv_f_tsd" list
```

README.md

@@ -320,7 +320,7 @@ optional arguments:
                         set the directory where decoder outputs will be stored
   -ne, --no-emoji       set to use plain text instead of emojis
   -tsd TEST_SUITES_DIR, --test-suites-dir TEST_SUITES_DIR
-                        set the directory where test suite will be read from
+                        set directory where test suite will be read from, multiple directories are supported with OS path separator (:)
 
 subcommands:
   {list,l,run,r,download,d,reference,f}

fluster/fluster.py

@@ -161,9 +161,14 @@ def __init__(
                 f" * output_dir: {self.output_dir}"
             )
 
+    def _walk_test_suite_dir(self):
+        for test_suite_dir in self.test_suites_dir.split(os.pathsep):
+            for root, dirnames, files in os.walk(test_suite_dir):
+                yield (root, dirnames, files)
+
     @lru_cache(maxsize=128)
     def _load_test_suites(self) -> None:
-        for root, _, files in os.walk(self.test_suites_dir):
+        for root, _, files in self._walk_test_suite_dir():
             for file in files:
                 if os.path.splitext(file)[1] == ".json":
                     try:

fluster/main.py

@@ -158,7 +158,10 @@ def _create_parser(self) -> argparse.ArgumentParser:
         parser.add_argument(
             "-tsd",
             "--test-suites-dir",
-            help="set the directory where test suite will be read from",
+            help=(
+                "set directory where test suite will be read from, "
+                f"multiple directories are supported with OS path separator ({os.pathsep})"
+            ),
             default=self.test_suites_dir,
         )
         subparsers = parser.add_subparsers(title="subcommands")