Fix --fail-under when SystemExit raised in script

Script can raises SystemExit when it running through exec function and code block with --fail-under never ran before.

Author: ownik

src/slipcover/__main__.py

@@ -299,8 +299,10 @@ def printit(coverage, outfile):
             code = sci.instrument(code)
 
         with sc.ImportManager(sci, file_matcher):
-            exec(code, script_globals)
-
+            try:
+                exec(code, script_globals)
+            except SystemExit as e:
+                return_code = e.code if e.code is not None else 0
     else:
         import runpy
         sys.argv = [*args.module, *args.script_or_module_args]

tests/branch.py

@@ -2,3 +2,5 @@ def foo(x):
     if x == 0:
         x += 1
 foo(0)
+
+raise SystemExit()

tests/test_coverage.py

@@ -694,14 +694,14 @@ def test_summary_in_output_zero_lines(do_branch):
 
 
 @pytest.mark.parametrize("json_flag", ["", "--json"])
-def test_fail_under(json_flag):
+def test_fail_under(tmp_path, json_flag):
     p = subprocess.run(f"{sys.executable} -m slipcover {json_flag} --fail-under 100 tests/branch.py".split(), check=False)
     assert 0 == p.returncode
 
-    p = subprocess.run(f"{sys.executable} -m slipcover {json_flag} --branch --fail-under 83 tests/branch.py".split(), check=False)
+    p = subprocess.run(f"{sys.executable} -m slipcover {json_flag} --branch --fail-under 85 tests/branch.py".split(), check=False)
     assert 0 == p.returncode
 
-    p = subprocess.run(f"{sys.executable} -m slipcover {json_flag} --branch --fail-under 84 tests/branch.py".split(), check=False)
+    p = subprocess.run(f"{sys.executable} -m slipcover {json_flag} --branch --fail-under 86 tests/branch.py".split(), check=False)
     assert 2 == p.returncode
 
     p = subprocess.run(f"{sys.executable} -m slipcover --branch --fail-under 93 -m pytest tests/pyt.py".split(), check=False)