TestHost can't check that SourceScripts is unmodified anymore because
C4ScriptHost contains a std::unique_ptr these days, so we can't copy
derived classes. Next best thing is just creating two instances of
TestHost and comparing those.
In the long term, there is no reason DirectExec should be concerned with
C4AulScript/C4ScriptHost. In the meantime, the lookup code from Fneval can
be moved into the function.
This allows eval in scenario script to access scenario script locals, but
that seems harmless.
The C4AulScript containing the source of the function was already mostly
used to get the relevant proplist or available from context. This will
allow more than one proplist plus the global one per scripthost to contain
functions.
GTest does not ship precompiled binaries anymore. They are now compiled
using the CMakelists.txt from gtest.
Add basic unit tests for C4Value, DirectExec and C4StringTable.