Update CI with testing framework from eigen_ci_cross_testing.

ci/scripts/test.linux.script.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+set -x
+
+# Enter build directory.
+rootdir=`pwd`
+cd ${EIGEN_CI_BUILDDIR}
+
+target=""
+if [[ ${EIGEN_CI_TEST_REGEX} ]]; then
+  target="-R ${EIGEN_CI_TEST_REGEX}"
+elif [[ ${EIGEN_CI_TEST_LABEL} ]]; then
+  target="-L ${EIGEN_CI_TEST_LABEL}"
+fi
+
+# Repeat tests up to three times to ignore flakes.  Do not re-run with -T test,
+# otherwise we lose test results for those that passed.
+# Note: starting with CMake 3.17, we can use --repeat until-pass:3, but we have
+#       no way of easily installing this on ppc64le.
+ctest --parallel ${NPROC} --output-on-failure --no-compress-output  \
+      --build-no-clean -T test ${target} || \
+  ctest -j${NPROC} --output-on-failure --no-compress-output --rerun-failed || \
+  ctest -j${NPROC} --output-on-failure --no-compress-output --rerun-failed
+
+# Return to root directory.
+cd ${rootdir}
+
+set +x