Don't use "make -j" when running tests

* test/infra/gitlab-ci.yml (.job-template):
* test/README: Don't use "make -j".

test/README

@@ -33,6 +33,10 @@ The following examples expect this directory as the current working
 directory.  If you call make from Emacs' root directory, use "make -C
 test" instead.
 
+Running several tests in parallel could result in unexpected side
+effects with ephemeral test errors.  Therefore, it is recommend not to
+use "make -j".
+
 The Makefile sets the environment variable $EMACS_TEST_DIRECTORY,
 which points to this directory.  This environment variable does not
 exist when the tests are run outside make.  The Makefile supports the

test/infra/gitlab-ci.yml

@@ -90,7 +90,6 @@ default:
      -e EMACS_TEST_JUNIT_REPORT=${EMACS_TEST_JUNIT_REPORT}
      -e EMACS_TEST_TIMEOUT=${EMACS_TEST_TIMEOUT}
      -e EMACS_TEST_VERBOSE=${EMACS_TEST_VERBOSE}
-     -e NPROC=`nproc`
      -e http_proxy=${http_proxy}
      -e https_proxy=${https_proxy}
      -e no_proxy=${no_proxy}
@@ -104,8 +103,8 @@ default:
         git diff --name-only FETCH_HEAD &&
         ( git diff --name-only FETCH_HEAD |
           xargs git checkout -f FETCH_HEAD ) &&
-        make -j \$NPROC &&
-        make -k -j \$NPROC ${make_params}"'
+        make &&
+        make -k ${make_params}"'
   after_script:
     # - docker ps -a
     # - pwd; printenv