Document usage of cli_test

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If you have multiple tests with the same names in multiple cli tests, e.g.

cli_test1.cpp

TEST_F(cli_test, some_test);

cli_test2.cpp

TEST_F(cli_test, some_test);

the cli_test will construct test directories for each test. These directories will have the same name. It might happen that one test runs its TearDown() (deleting the directory) while the other test runs -> tests fail with cryptic errors.

One should create a new fixture for each cli_test:

cli_test.hpp

struct cli_test1: public cli_test {};
struct cli_test2: public cli_test {};

cli_test1.cpp

TEST_F(cli_test1, some_test);

cli_test2.cpp

TEST_F(cli_test2, some_test);