Enable use of a unit test thread limit in Jenkins. (mantidproject#39481)

Add a check for a new `UNIT_TEST_THREADS` environment variable that can
be set in the Jenkins configuration for each node that can be used to
limit the number of unit tests to be run in parallel. This has been done
so that we can limit the number of unit tests processes on out macOS
builders, some of which have limited RAM (compared to the core count).

### Further detail of work
I have set `UNIT_TEST_THREADS=6` on the following nodes, which have been
observed to timeout on tests:
- isis-ndw2564
- isis-ndw2995
- isis-ndw2996

This value is two below the number of cores, so will hopefully give the
machines more resources to run the tests in parallel successfully. We
can tweak the number inside the Jenkins configuration if required.

Author: thomashampson

buildconfig/Jenkins/Conda/run-tests

@@ -26,6 +26,13 @@ if [[ -z "$BUILD_THREADS_SYSTEM_TESTS" ]]; then
     BUILD_THREADS_SYSTEM_TESTS=3
 fi
 
+# Jenkins nodes may specify the maximum number of unit tests to run in parallel.
+# This is useful to prevent test timeouts for machines with limited RAM.
+if [[ -z "$UNIT_TEST_THREADS" ]]; then
+    # Default is number of build threads.
+    UNIT_TEST_THREADS=$BUILD_THREADS
+fi
+
 function run_with_xvfb {
     if [ $(command -v xvfb-run) ]; then
         # Use -e because a bug on RHEL7 means --error-file produces an error: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=337703;msg=2
@@ -85,7 +92,7 @@ cd $WORKSPACE/build
 # mkdir for test logs
 mkdir -p test_logs
 if [[ $ENABLE_UNIT_TESTS == true ]]; then
-    run_with_xvfb ctest -j$BUILD_THREADS --no-compress-output -T Test -O test_logs/ctest_$OSTYPE.log --schedule-random --output-on-failure --repeat until-pass:3 $WINDOWS_TEST_OPTIONS $WINDOWS_UNITTEST_TIMEOUT_OPTIONS
+    run_with_xvfb ctest -j$UNIT_TEST_THREADS --no-compress-output -T Test -O test_logs/ctest_$OSTYPE.log --schedule-random --output-on-failure --repeat until-pass:3 $WINDOWS_TEST_OPTIONS $WINDOWS_UNITTEST_TIMEOUT_OPTIONS
 fi
 
 if [[ $ENABLE_DOCS == true && $ENABLE_DOC_TESTS == true ]]; then