Add simple C++ test

[shadinua]

Builds a primitive C++ application using themis and soter libraries located at standard system paths. This allows us to check that Themis library and Themis C++ development environment have been properly installed on the target system.

# Build & run test
make test_cpp_simple
# Clean
make clean_themispp_simple_test

Yes, naming looks a bit strange, but now this is the approach taken in Makefiles. Hope we can improve it someday.

[ilammy]

LGTM 👍

But I still wonder if we are better off integrating docs/examples into the main Makefile. Some day.

(Also, it would be nice to get CircleCI/GHA to build this target too.)

[ilammy]

Now I remember that RustThemis has some special relationship with included Makefiles, but this should be okay because tests/test.mk is not included when building by Cargo:

themis/Makefile

Lines 363 to 370 in a558d85

	ifndef CARGO
	include src/wrappers/themis/jsthemis/jsthemis.mk
	include src/wrappers/themis/themispp/themispp.mk
	include src/wrappers/themis/wasm/wasmthemis.mk
	include jni/themis_jni.mk
	include tests/test.mk
	include tools/afl/fuzzy.mk
	endif