tests: Set test concurrency in the CMake cache

By using a CMake cache variable, it allows developers to easily change
the default of 2 with CMake via

    ./test.py -c 4

or

    cmake -DSeastar_UNIT_TEST_SMP=4 .

test.py

@@ -29,6 +29,7 @@
     parser.add_argument('--mode', choices=seastar_cmake.SUPPORTED_MODES, help="Run only tests for given build mode")
     parser.add_argument('--timeout', action="store",default="300",type=int, help="timeout value for test execution")
     parser.add_argument('--jenkins', action="store",help="jenkins output file prefix")
+    parser.add_argument('--smp', '-c', action="store",default='2',type=int,help="Number of threads for multi-core tests")
     parser.add_argument('--verbose', '-v', action = 'store_true', default = False,
                         help = 'Verbose reporting')
     args = parser.parse_args()
@@ -44,6 +45,7 @@ def run_tests(mode):
         TRANSLATED_CMAKE_ARGS = [
             tr(args.timeout, 'TEST_TIMEOUT'),
             tr(args.fast, 'EXECUTE_ONLY_FAST_TESTS'),
+            tr(args.smp, 'UNIT_TEST_SMP'),
             tr(args.jenkins, 'JENKINS', value_when_none=''),
         ]
 

tests/unit/CMakeLists.txt

@@ -23,6 +23,12 @@
 # Logical target for all unit tests.
 add_custom_target (unit_tests)
 
+set (Seastar_UNIT_TEST_SMP
+  2
+  CACHE
+  STRING
+  "Run unit tests with this many cores.")
+
 #
 # Define a new unit test with the given name.
 #
@@ -61,9 +67,6 @@ function (seastar_add_test name)
     BOOST
     CUSTOM)
 
-  # Run all Seastar tests on two cores by default.
-  set (num_cores 2)
-
   cmake_parse_arguments (parsed_args
     ""
     "WORKING_DIRECTORY;KIND"
@@ -97,7 +100,7 @@ function (seastar_add_test name)
         list (APPEND args ${jenkins_args})
       endif ()
 
-      list (APPEND args -- -c ${num_cores})
+      list (APPEND args -- -c ${Seastar_UNIT_TEST_SMP})
       list (APPEND args ${parsed_args_RUN_ARGS})
     elseif (parsed_args_KIND STREQUAL "BOOST")
       set (args "")