replace tempdir by tempfile (#2143)

## Issue Addressed

Fixes #2141 
Remove [tempdir](https://docs.rs/tempdir/0.3.7/tempdir/) in favor of [tempfile](https://docs.rs/tempfile/3.1.0/tempfile/).

## Proposed Changes

`tempfile` has a slightly different api that makes creating temp folders with a name prefix a chore (`tempdir::TempDir::new("toto")` => `tempfile::Builder::new().prefix("toto").tempdir()`).

So I removed temp folder name prefix where I deemed it not useful.

Otherwise, the functionality is the same.

slasher/tests/attester_slashings.rs

@@ -6,7 +6,7 @@ use slasher::{
     Config, Slasher,
 };
 use std::collections::HashSet;
-use tempdir::TempDir;
+use tempfile::tempdir;
 use types::{AttesterSlashing, Epoch, IndexedAttestation};
 
 #[test]
@@ -169,7 +169,7 @@ fn slasher_test(
     current_epoch: u64,
     should_process_after: impl Fn(usize) -> bool,
 ) {
-    let tempdir = TempDir::new("slasher").unwrap();
+    let tempdir = tempdir().unwrap();
     let config = Config::new(tempdir.path().into());
     let slasher = Slasher::open(config, logger()).unwrap();
     let current_epoch = Epoch::new(current_epoch);
@@ -196,7 +196,7 @@ fn parallel_slasher_test(
     expected_slashed_validators: HashSet<u64>,
     current_epoch: u64,
 ) {
-    let tempdir = TempDir::new("slasher").unwrap();
+    let tempdir = tempdir().unwrap();
     let config = Config::new(tempdir.path().into());
     let slasher = Slasher::open(config, logger()).unwrap();
     let current_epoch = Epoch::new(current_epoch);

slasher/tests/proposer_slashings.rs

@@ -2,12 +2,12 @@ use slasher::{
     test_utils::{block as test_block, logger, E},
     Config, Slasher,
 };
-use tempdir::TempDir;
+use tempfile::tempdir;
 use types::{Epoch, EthSpec};
 
 #[test]
 fn empty_pruning() {
-    let tempdir = TempDir::new("slasher").unwrap();
+    let tempdir = tempdir().unwrap();
     let config = Config::new(tempdir.path().into());
     let slasher = Slasher::<E>::open(config.clone(), logger()).unwrap();
     slasher.prune_database(Epoch::new(0)).unwrap();
@@ -17,7 +17,7 @@ fn empty_pruning() {
 fn block_pruning() {
     let slots_per_epoch = E::slots_per_epoch();
 
-    let tempdir = TempDir::new("slasher").unwrap();
+    let tempdir = tempdir().unwrap();
     let mut config = Config::new(tempdir.path().into());
     config.chunk_size = 2;
     config.history_length = 2;

slasher/tests/random.rs

@@ -8,7 +8,7 @@ use slasher::{
     Config, Slasher,
 };
 use std::cmp::max;
-use tempdir::TempDir;
+use tempfile::tempdir;
 use types::{Epoch, EthSpec};
 
 #[derive(Debug)]
@@ -38,7 +38,7 @@ fn random_test(seed: u64, test_config: TestConfig) {
     println!("Running with seed {}", seed);
     let mut rng = StdRng::seed_from_u64(seed);
 
-    let tempdir = TempDir::new("slasher").unwrap();
+    let tempdir = tempdir().unwrap();
 
     let mut config = Config::new(tempdir.path().into());
     config.validator_chunk_size = 1 << rng.gen_range(1, 4);

slasher/tests/wrap_around.rs

@@ -2,12 +2,12 @@ use slasher::{
     test_utils::{indexed_att, logger},
     Config, Error, Slasher,
 };
-use tempdir::TempDir;
+use tempfile::tempdir;
 use types::Epoch;
 
 #[test]
 fn attestation_pruning_empty_wrap_around() {
-    let tempdir = TempDir::new("slasher").unwrap();
+    let tempdir = tempdir().unwrap();
     let mut config = Config::new(tempdir.path().into());
     config.validator_chunk_size = 1;
     config.chunk_size = 16;
@@ -41,7 +41,7 @@ fn attestation_pruning_empty_wrap_around() {
 // Test that pruning can recover from a `MapFull` error
 #[test]
 fn pruning_with_map_full() {
-    let tempdir = TempDir::new("slasher").unwrap();
+    let tempdir = tempdir().unwrap();
     let mut config = Config::new(tempdir.path().into());
     config.validator_chunk_size = 1;
     config.chunk_size = 16;