Use inline Java in assert pattern test (#1248)

Co-authored-by: Marinaaaa <gabidolboebka@gmail.com>

Author: Beeeeeeee-c

test/patterns/assert_in_code/Book.java

@@ -1,19 +1,24 @@
 # SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
 # SPDX-License-Identifier: MIT
 
-import os.path
-from pathlib import Path
+from textwrap import dedent
 from unittest import TestCase
 
 from aibolit.patterns.assert_in_code.assert_in_code import AssertInCode
 from aibolit.ast_framework import AST
-from aibolit.utils.ast_builder import build_ast
+from aibolit.utils.ast_builder import build_ast_from_string
 
 
 class AssertInCodeTestCase(TestCase):
-    cur_file_dir = Path(os.path.realpath(__file__)).parent
-
     def test_assert_in_code(self):
-        file = Path(self.cur_file_dir, 'Book.java')
-        ast = AST.build_from_javalang(build_ast(file))
-        self.assertEqual(AssertInCode().value(ast), [6])
+        content = dedent(
+            '''\
+            class Book {
+              void foo(String x) {
+                assert x != null; // here
+              }
+            }
+            '''
+        )
+        ast = AST.build_from_javalang(build_ast_from_string(content))
+        self.assertEqual(AssertInCode().value(ast), [3])