Update Secure Cell test suite with the tests used in WasmThemis. There
are some minor tweaks here and there to make them compatible, but it's
all the same tests otherwise.

Instead of throwing a generic error with INVALID_PARAMETER status code,
throw a proper TypeError when we notice an issue with input types.

This commit updates all cryptosystems, not only Secure Cell.

In addition to allowing empty buffers and omitted arguments, accept
explicit "null" values when the context is not used. This improves
API compatibility with WasmThemis.

In JsThemis there is no SecureMessageSign and SecureMessageVerify
objects, empty keys should be used instead.

Allow omitted keys to be specified with "null" values in addition to
empty buffers. This improves compatibility with WasmThemis API and is
consistent with other places where we allow optional values.

Check that the user has provided either public or private key, do not
allow both keys to be omitted.

Note that JsThemis uses a bit different naming and does not have a
"destroy()" method. The Secure Comparator object is freed when Node.js
runtime collects the garbage.

API is slightly different in JsThemis and WasmThemis, the names are not
the same and JsThemis does not have a dedicated negotiation method,
using just unwrap() for that.

WasmThemis verifies the private key during Secure Session construction.
Don't defer this to usage in JsThemis either.

We should check the call result as it may be empty if the callback
throws an exception. Failure to check for this leads to process abort:

    FATAL ERROR: v8::ToLocalChecked Empty MaybeLocal.
     1: 0x8fa090 node::Abort() [node]
     2: 0x8fa0dc  [node]
     3: 0xb0030a v8::Utils::ReportApiFailure(char const*, char const*) [node]
     4: 0x7ff9182b1785 jsthemis::SecureSession::get_public_key_for_id_callback(void const*, unsigned long, void*, unsigned long, void*)
     [...]
     9: 0xb8e96f  [node]
    10: 0xb8f4d9 v8::internal::Builtin_HandleApiCall(int, v8::internal::Object**, v8::internal::Isolate*) [node]
    11: 0x1762fd45be1d
    Aborted (core dumped)

We don't want this to happen.

Since we're publishing test code on npm now, we should properly
attribute this file. The tests were added in 944b9e0 ("Add JSTHEMIS
test", 2016-01-17).

We know that JsThemis returns results as instances of Node.js Buffer so
we can verify that signed message includes the plaintext by using the
"includes" method. We don't need to write it ourselves.