test: add unit tests for core, parser and operations

tests/test_exceptions.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+from sshctl.core.exceptions import (
+    ConfigError,
+    ParseError,
+    ResolverError,
+    SshctlError,
+)
+
+
+def test_exception_hierarchy() -> None:
+    assert issubclass(ConfigError, SshctlError)
+    assert issubclass(ParseError, SshctlError)
+    assert issubclass(ResolverError, SshctlError)
+
+
+def test_sshctl_error_is_base() -> None:
+    assert issubclass(SshctlError, Exception)
+
+
+def test_exception_raised() -> None:
+    try:
+        raise ConfigError("test error")
+    except SshctlError as e:
+        assert str(e) == "test error"