message: Replace ffi_error_ with backend_err_ in test names
diff: @@ -299,7 +299,7 @@ mod tests {
  // constructors
  #[test]
  - fn ffi_error_foreign_panic() {
  + fn backend_err_foreign_panic() {
  let error = BackendError::foreign_panic();
  match error {
  BackendError::ForeignPanic { .. } => {}
  @@ -308,7 +308,7 @@ mod tests {
  }
  #[test]
  - fn ffi_error_bad_argument() {
  + fn backend_err_bad_argument() {
  let error = BackendError::bad_argument();
  match error {
  BackendError::BadArgument { .. } => {}
  @@ -326,7 +326,7 @@ mod tests {
  }
  #[test]
  - fn ffi_error_out_of_gas() {
  + fn backend_err_out_of_gas() {
  let error = BackendError::out_of_gas();
  match error {
  BackendError::OutOfGas { .. } => {}
  @@ -335,7 +335,7 @@ mod tests {
  }
  #[test]
  - fn ffi_error_unknown() {
  + fn backend_err_unknown() {
  let error = BackendError::unknown("broken");
  match error {
  BackendError::Unknown { msg, .. } => assert_eq!(msg, "broken"),
  @@ -344,7 +344,7 @@ mod tests {
  }
  #[test]
  - fn ffi_error_user_err() {
  + fn backend_err_user_err() {
  let error = BackendError::user_err("invalid input");
  match error {
  BackendError::UserErr { msg, .. } => assert_eq!(msg, "invalid input"),
  