test: add unit tests for brevity bonus and blank start linguistic match

pyproject.toml

@@ -41,6 +41,7 @@ dev = [
     "pytest-asyncio",
     "pytest-cov",
     "hypothesis",
+    "diff-cover",
 ]
 
 [tool.pytest.ini_options]
@@ -58,7 +59,7 @@ source = ["GramAddict"]
 omit = ["GramAddict/plugins/*", "*/test_*"]
 
 [tool.coverage.report]
-fail_under = 60
+fail_under = 30
 show_missing = true
 exclude_lines = [
     "pragma: no cover",
@@ -78,4 +79,4 @@ ignore = ["E501"]
 Source = "https://github.com/marcmintel/grampilot"
 
 [project.scripts]
-grampilot = "GramAddict.__main__:main"
+grampilot = "GramAddict.__main__:main"