Add stress unit tests with several processes to config

tests/stress_tests/.automation/unittests/nightly_configs/desktop_test_config.xml

@@ -1,6 +1,7 @@
 <attributes>
     <processes>
         <value>1</value>
+        <value>2</value>
     </processes>
     <threads>
         <value>1</value>

tests/stress_tests/.automation/unittests/nightly_configs/myriad_test_config.xml

@@ -1,6 +1,7 @@
 <attributes>
     <processes>
         <value>1</value>
+        <value>2</value>
     </processes>
     <threads>
         <value>1</value>

tests/stress_tests/.automation/unittests/weekly_configs/desktop_test_config.xml

@@ -1,6 +1,8 @@
 <attributes>
     <processes>
         <value>1</value>
+        <value>2</value>
+        <value>8</value>
     </processes>
     <threads>
         <value>1</value>
@@ -17,4 +19,4 @@
         <model name="alexnet" precision="FP32" source="omz" />
         <model name="mobilenet-ssd" precision="FP32" source="omz" />
     </models>
-</attributes>
+</attributes>