Restrict the OpenMP threads # gitlab-ci tests uses

A single gitlab-runner can run multiple test stages
concurrently on the same hardware. When we have 3
jobs asking for 100% of the cpu's we get a 300%
reduction in performance caused by task switching

.gitlab/ci/ubuntu1604.yml

@@ -90,7 +90,6 @@ test:ubuntu1604_gcc48:
     #mpi location so that `built_against_test_install` tests
     #pass
     CMAKE_PREFIX_PATH: "/opt/openmpi/"
-
   dependencies:
     - build:ubuntu1604_gcc48
   needs:
@@ -127,7 +126,6 @@ test:ubuntu1604_clang5:
     - .ubuntu1604
     - .cmake_test_linux
     - .only-default
-
   dependencies:
     - build:ubuntu1604_clang5
   needs:

.gitlab/ci/ubuntu1804.yml

@@ -28,6 +28,10 @@ test:ubuntu1804_gcc9:
     - .ubuntu1804
     - .cmake_test_linux
     - .only-default
+  variables:
+    #Restrict OpenMP number of threads since multiple test stages
+    #execute on the same hardware concurrently
+    OMP_NUM_THREADS: 4
   dependencies:
     - build:ubuntu1804_gcc9
   needs:
@@ -98,6 +102,10 @@ test:ubuntu1804_gcc7_2:
     - .ubuntu1804
     - .cmake_test_linux
     - .only-master
+  variables:
+    #Restrict OpenMP number of threads since multiple test stages
+    #execute on the same hardware concurrently
+    OMP_NUM_THREADS: 4
   dependencies:
     - build:ubuntu1804_gcc7_2
   needs:
@@ -131,6 +139,10 @@ test:ubuntu1804_gcc6:
     - .ubuntu1804
     - .cmake_test_linux
     - .only-master
+  variables:
+    #Restrict OpenMP number of threads since multiple test stages
+    #execute on the same hardware concurrently
+    OMP_NUM_THREADS: 3
   dependencies:
     - build:ubuntu1804_gcc6
   needs: