LibCore+Tests: Add SeekableStream::truncate()

Author: https://github.com/AtkinsSJ
Commit: 4d5080388a
Pull-request: https://github.com/SerenityOS/serenity/pull/12317
Reviewed-by: https://github.com/sin-ack ✅
Reviewed-by: https://github.com/trflynn89

Tests/LibCore/TestLibCoreStream.cpp

@@ -135,6 +135,18 @@ TEST_CASE(file_adopt_invalid_fd)
     EXPECT_EQ(maybe_file.error().code(), EBADF);
 }
 
+TEST_CASE(file_truncate)
+{
+    auto maybe_file = Core::Stream::File::open("/tmp/file-truncate-test.txt", Core::Stream::OpenMode::Write);
+    auto file = maybe_file.release_value();
+
+    EXPECT(!file->truncate(999).is_error());
+    EXPECT_EQ(file->size().release_value(), 999);
+
+    EXPECT(!file->truncate(42).is_error());
+    EXPECT_EQ(file->size().release_value(), 42);
+}
+
 // TCPSocket tests
 
 TEST_CASE(should_error_when_connection_fails)