Unset upfront_shutdown_script using bit clearing

The test_upfront_shutdown_script functional test clears this feature
flag. However, the method used to clear the flag is implemented by bit
toggling. Thus, if the flag is not set the method would actually set it.
Implement the method using bit clearing instead.

Author: jkczyz

lightning/src/ln/features.rs

@@ -272,7 +272,7 @@ impl<T: sealed::UpfrontShutdownScript> Features<T> {
 	}
 	#[cfg(test)]
 	pub(crate) fn unset_upfront_shutdown_script(&mut self) {
-		self.flags[0] ^= 1 << 5;
+		self.flags[0] &= !(1 << 5);
 	}
 }