Use a fixed number of omp threads for unit tests on builders

The default in conjunction with ctest -j produces a large number
of threads that compete with each other and give worse performance.

buildconfig/Jenkins/buildscript

@@ -342,7 +342,8 @@ fi
 userconfig_dir=$HOME/.mantid
 rm -fr $userconfig_dir
 mkdir -p $userconfig_dir
-touch $userconfig_dir/Mantid.user.properties
+# use a fixed number of openmp threads to avoid overloading the system
+echo MultiThreaded.MaxCores=2 > $userconfig_dir/Mantid.user.properties
 
 if [[ ${DO_UNITTESTS} == true ]]; then
   $CTEST_EXE -j${BUILD_THREADS:?} --schedule-random --output-on-failure

buildconfig/Jenkins/buildscript.bat

@@ -170,7 +170,9 @@ del %USERPROPS%
 set CONFIGDIR=%APPDATA%\mantidproject\mantid
 rmdir /S /Q %CONFIGDIR%
 mkdir %CONFIGDIR%
-call cmake.exe -E touch %USERPROPS%
+:: use a fixed number of openmp threads to avoid overloading the system
+echo MultiThreaded.MaxCores=2 > %USERPROPS%
+
 call ctest.exe -C %BUILD_CONFIG% -j%BUILD_THREADS% --schedule-random --output-on-failure
 if ERRORLEVEL 1 exit /B %ERRORLEVEL%