[fix][build] build pulsar-client-cpp with BUILD_TESTS=OFF

[rickif]

Fixes https://github.com/apache/pulsar/issues/1964

Motivation

When the pulsar-client-cpp is built with cmake . -DTESTS=OFF, the cmake stil would look for the GTEST_INCLUDE_PATH and GMOCK_INCLUDE_PATH, which leads the problem in #1964.

Modifications

• remove gtest/gmock from the include_directories of CMakeLists.txt when the BUILD_TESTS=OFF.

Verifying this change

• [x] Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• [ ] Dependencies (add or upgrade a dependency)
• [ ] The public API
• [ ] The schema
• [ ] The default values of configurations
• [ ] The binary protocol
• [ ] The REST endpoints
• [ ] The admin CLI options
• [x] Anything that affects deployment

Documentation

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Matching PR in forked repository

PR in forked repository: https://github.com/rickif/pulsar