Make lightning-background-processor test failures more debugable

Instead of asserting a `Result` `is_ok`, we should always simply
`unwrap` to get a backgrace, and we should avoid doing so if the
thread is already panicking.

Author: TheBlueMatt

lightning-background-processor/src/lib.rs

@@ -1208,7 +1208,9 @@ mod tests {
 			}
 		}
 
-		assert!(bg_processor.stop().is_ok());
+		if !std::thread::panicking() {
+			bg_processor.stop().unwrap();
+		}
 	}
 
 	#[test]
@@ -1326,7 +1328,9 @@ mod tests {
 			_ => panic!("Unexpected event: {:?}", event),
 		}
 
-		assert!(bg_processor.stop().is_ok());
+		if !std::thread::panicking() {
+			bg_processor.stop().unwrap();
+		}
 	}
 
 	#[test]
@@ -1345,7 +1349,9 @@ mod tests {
 			}
 		}
 
-		assert!(bg_processor.stop().is_ok());
+		if !std::thread::panicking() {
+			bg_processor.stop().unwrap();
+		}
 	}
 
 	#[test]
@@ -1519,6 +1525,8 @@ mod tests {
 			_ => panic!("Unexpected event"),
 		}
 
-		assert!(bg_processor.stop().is_ok());
+		if !std::thread::panicking() {
+			bg_processor.stop().unwrap();
+		}
 	}
 }