Improve test coverage of os.open

tests/snippets/builtin_open.py

@@ -0,0 +1,8 @@
+fd = open('README.md')
+assert 'RustPython' in fd.read()
+
+try:
+    open('DoesNotExist')
+    assert False
+except FileNotFoundError:
+    pass

tests/snippets/os_open.py

@@ -2,3 +2,9 @@ import os
 
 assert os.open('README.md', 0) > 0
 
+
+try:
+    os.open('DOES_NOT_EXIST', 0)
+    assert False
+except FileNotFoundError:
+    pass

vm/src/builtins.rs

@@ -767,6 +767,11 @@ pub fn make_module(ctx: &PyContext) -> PyObjectRef {
     ctx.set_attr(&py_mod, "ValueError", ctx.exceptions.value_error.clone());
     ctx.set_attr(&py_mod, "IndexError", ctx.exceptions.index_error.clone());
     ctx.set_attr(&py_mod, "ImportError", ctx.exceptions.import_error.clone());
+    ctx.set_attr(
+        &py_mod,
+        "FileNotFoundError",
+        ctx.exceptions.file_not_found_error.clone(),
+    );
     ctx.set_attr(
         &py_mod,
         "StopIteration",