Add option for CI to run without building unit tests

[nlslatt]

What Needs to be Done?

The version of Google Test we bundle with vt does not build with the Intel 18 compiler. Add an option to the CI scripts to allow skipping the build of gtest and anything that depends on gtest (i.e., the unit tests).

[nlslatt]

We decided to handle this another way, by grabbing an older version of googletest using a modified version of the fetch-googletest.sh script included in tests/extern/. The commands below were used:

pushd $CI_PROJECT_DIR/tests/extern && perl -pe "s/GTEST_BRANCHTAG=.*$/GTEST_BRANCHTAG=release-1.10.0/" fetch-googletest.sh > fetch-old-googletest.sh && chmod a+x fetch-old-googletest.sh && ./fetch-old-googletest.sh
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