random: Fix "nonminimal-bool" clippy warning

The error shown below was caught during a dependency bump in the CCv0
branch, but we better fix it here first.
```
error: this boolean expression can be simplified
  --> src/random.rs:85:21
   |
85 |             assert!(!ret.is_ok());
   |                     ^^^^^^^^^^^^ help: try: `ret.is_err()`
   |
   = note: `-D clippy::nonminimal-bool` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#nonminimal_bool

error: this boolean expression can be simplified
  --> src/random.rs:93:17
   |
93 |         assert!(!ret.is_ok());
   |                 ^^^^^^^^^^^^ help: try: `ret.is_err()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#nonminimal_bool
```

Fixes: #4523

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

src/agent/src/random.rs

@@ -82,7 +82,7 @@ mod tests {
         if nix::unistd::Uid::effective().is_root() {
             assert!(ret.is_ok());
         } else {
-            assert!(!ret.is_ok());
+            assert!(ret.is_err());
         }
     }
 
@@ -90,6 +90,6 @@ mod tests {
     fn test_reseed_rng_zero_data() {
         let seed = [];
         let ret = reseed_rng(&seed);
-        assert!(!ret.is_ok());
+        assert!(ret.is_err());
     }
 }