Make cpptest build on Ubuntu

[apivovarov]

Changes in code:

1. add dl to target_link_libraries in CMakeLists.txt for Tests to solve undefined reference to dl* issue

libtvm.so: undefined reference to `dlopen'
libtvm.so: undefined reference to `dlclose'
libtvm.so: undefined reference to `dlerror'
libtvm.so: undefined reference to `dlsym'

https://discuss.tvm.ai/t/make-cpptest-failed

Changes in docs:

1. add apt command to install gtest
2. Add USE_LLVM=ON param to cmake because llvm target should be enabled for one of the tests
3. replace make -j with make -j$(nproc) because just -j starts building for all files in parallel (list of files is quite big).
4. remove || exit -1 from test loop.
   exit -1 makes it exists from the shell where you run the command in case of test failure.
   We might want to break the loop but not to exist from the shell.
   Anyway, usual junit practice is to run all tests and check the results.