Add tests for Package functions

[erikscheurer]

This PR adds tests for the most of the package functions. The tests are heavily inspired by the python bindings.

The tests use the original Julia bindings, but make Julia use a dummy API instead of the real preCICE API.

[erikscheurer]

At the moment, the fake Solverdummy interface has to be specified manually for the test. This is done by running in the test directory:

$ make
$ julia
julia> ]
pkg> dev ../
julia> push!(Libc.Libdl.DL_LOAD_PATH, "<path-to-test-directory>")
julia> ]
pkg> test PreCICE

Then the new testset should run, and pass except for one test. At the moment, after this testset I've put in a exit() so the solverdummy doesn't run, because running the solverdummy would have to access the correct SolverInterface. Until the path works, I'll leave it like this.
The one test that doesn't work is the readWriteScalarData test. Here a scalar value is written to the fake_read_write_buffer. This works without problem as far as my debugging shows.
However, reading the data doesn't. The Problem probably lies somewhere in handling references and pointers with the C code.
PreCICE.jl passes a reference to a double into the C function which then is supposed to rewrite the data where the pointer is pointing to.
In the Python bindings, the pointer is different, but this is C++ code instead of C and I don't have enough experience with C/C++ to fix this error.
The tests should also resemble how it is implemented in the original SolverInterface so the orientation shouldn't only be the python bindings.

[IshaanDesai]

Only a few minor comments, otherwise its good to go 👍 Good job here, we now have a proper testing infrastructure!