Set OMP vars before importing numpy when testing (#1007)

**Context:** When testing Kokkos the OMP env vars were not having an
effect because they were set after importing numpy, and were only set
for one test file.

**Description of the Change:** Set the OMP env vars at the conftest.py
file and before importing numpy.

**Benefits:** Faster testing times

**Possible Drawbacks:** The number of threads is fixed to 2. Different
machines might support larger numbers and increase performance.

frontend/test/conftest.py

@@ -14,7 +14,14 @@
 """
 Pytest configuration file for Catalyst test suite.
 """
-# pylint: disable=unused-import
+
+import os
+
+# OMP env vars have to be set before importing numpy in order to have an effect
+os.environ["OMP_PROC_BIND"] = "false"
+os.environ["OMP_NUM_THREADS"] = "2"
+
+# pylint: disable=unused-import,wrong-import-position
 import platform
 
 import numpy as np

frontend/test/pytest/test_mid_circuit_measurement.py

@@ -13,7 +13,6 @@
 # limitations under the License.
 """Tests for mid-circuit measurements in Catalyst"""
 
-import os
 from dataclasses import asdict
 from functools import reduce
 from typing import Iterable, Sequence
@@ -29,8 +28,6 @@
 from catalyst import CompileError, cond, measure, qjit
 
 # TODO: add tests with other measurement processes (e.g. qml.sample, qml.probs, ...)
-os.environ["OMP_PROC_BIND"] = "false"
-os.environ["OMP_NUM_THREADS"] = "2"
 
 # pylint: disable=too-many-public-methods