Fix perf context test (rust-rocksdb#321)

Fixing perf context test failure with `cargo test -- --test_threads=1`. The test assume default perf level is `PerfLevel::EnableCount`, which apparently will affected by other tests.

Tested by running `cargo test -- --test_threads=1`.

Closing rust-rocksdb#318 

Signed-off-by: Yi Wu <yiwu@pingcap.com>

src/perf_context.rs

@@ -419,6 +419,7 @@ mod test {
             }
         }
 
+        set_perf_level(PerfLevel::EnableCount);
         let mut ctx = PerfContext::get();
 
         let mut iter = db.iter();