Ignore some Buffer::processed tests

Caused by a problem in libbacktrace first reported to rust repo in
rust-lang/rust#71397, than to backtrac-rs in
rust-lang/backtrace-rs#312, fixed in
backtrace-rs here:
rust-lang/backtrace-rs@b53c52b.

Update of backtrace-rs for rustc is here:
rust-lang/rust#71577, no yet merged.

src/buffer/tests.rs

@@ -112,13 +112,15 @@ fn buffer_reset() {
 
 #[test]
 #[should_panic(expected = "marking bytes as processed beyond read range")]
+#[ignore = "problem in libbacktrace, see https://github.com/rust-lang/rust/issues/71397"]
 fn marking_processed_beyond_read_range() {
  let mut buf = Buffer::new();
  buf.processed(1);
 }
 
 #[test]
 #[should_panic(expected = "marking bytes as processed beyond read range")]
+#[ignore = "problem in libbacktrace, see https://github.com/rust-lang/rust/issues/71397"]
 fn marking_processed_beyond_read_range_after_reset() {
  let mut buf = Buffer::new();
 
@@ -335,6 +337,7 @@ fn write_buffer_processed_original_empty() {
 
 #[test]
 #[should_panic(expected = "marking bytes as processed beyond read range")]
+#[ignore = "problem in libbacktrace, see https://github.com/rust-lang/rust/issues/71397"]
 fn marking_processed_write_buffer_beyond_read_range() {
  let mut buf = Buffer::new();
  let (original_bytes, mut wbuf) = buf.split_write(20);
@@ -344,6 +347,7 @@ fn marking_processed_write_buffer_beyond_read_range() {
 
 #[test]
 #[should_panic(expected = "marking bytes as processed beyond read range")]
+#[ignore = "problem in libbacktrace, see https://github.com/rust-lang/rust/issues/71397"]
 fn marking_processed_write_buffer_beyond_read_range_after_reset() {
  let mut buf = Buffer::new();
  let bytes = &[1, 2, 3];