Compile integration_testing.rs conditionally

[muzarski]

Fixes: #285

After the change we have:

$ cmake .. && make -j 8
...
$ objdump -TC libscylla-cpp-driver.so | grep testing_ 
<empty output>

$ cmake -DCASS_BUILD_INTEGRATION_TESTS=ON .. && make -j 8
...
$ objdump -TC libscylla-cpp-driver.so | grep testing_
00000000000eed40 g    DF .text	000000000000005e  Base        testing_cluster_get_connect_timeout
00000000000eeda0 g    DF .text	0000000000000052  Base        testing_cluster_get_port
00000000000eee00 g    DF .text	00000000000002b8  Base        testing_cluster_get_contact_points
00000000000ef0c0 g    DF .text	0000000000000026  Base        testing_free_contact_points
00000000000ef190 g    DF .text	00000000000000ee  Base        testing_statement_set_sleeping_history_listener

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• PR description sums up the changes and reasons why they should be introduced.
• [ ] I have implemented Rust unit tests for the features/changes introduced.
• [ ] I have enabled appropriate tests in .github/workflows/build.yml in gtest_filter.
• [ ] I have enabled appropriate tests in .github/workflows/cassandra.yml in gtest_filter.