When trying to run the dotnet sample the following error happened:

```
FailFast:
Couldn't find a valid ICU package installed on the system. Set the
configuration flag System.Globalization.Invariant to true if you want to
run with no globalization support.
```

By adding this file, the configuration flag mentioned in the error
message is set to true, and the sample works again. [1]

[1] dotnet/core#2186 (comment)

The helloworld demo used a really complicated system to build the elf
file on the host and then transfer it to the docker image. It's better
to simply build it inside docker, as the test does.

This commit copies Dockerfile, Makefile and helloworld.c from the test
to the sample. The Makefile has been modified to still use
enclave_config.json (the test wasn't using it).

When the number of ethreads is 1 the java app hangs after printing the
message. With a higher number of ethreads it exits as expected.

The scripts are a mix of bash and a bit of GNU expect for flexibility,
as each sample needs to be tested in slightly different ways.

Each sample must have a test.sh script in its folder so that the CI
detects it and runs it. They are executed by run_sample.sh script,
created using run_test.sh as an example.

This commit adds several samples to the CI, but not all of them:

- The ml folder samples, for example, take far too long to be part of a
  regular CI run, and need caching of docker images, which is a task on
  its own.

- The openmp and nodejs samples seem to be broken.