Create test to verify correct functioning of ptys in system call

[ryandesign (Ryan Carsten Schmidt)]

I would like to create a test to verify the correct functioning of ptys in system calls—to verify that the changes in 8765c14223c2826c958cddd4e33c17801d63ddb4/macports-base work and continue working.

I do not understand our test system so I'm having trouble doing this.

I tried the attached patch. It tries to run expect -c "spawn echo" and then verify that the output is indeed "spawn echo". It doesn't work for two reasons:

1. The output seemingly has whitespace at the end. Being output from terminal, it should have a carriage return and a newline at the end. [string trim] should be removing those, but isn't, for reasons I don't understand.
2. The expected output is produced, even if the sandbox fix from 8765c14223c2826c958cddd4e33c17801d63ddb4/macports-base is reverted, which suggests the test isn't being run under the sandbox and is therefore useless for verifying correct functioning within the sandbox.

[raimue (Rainer Müller)]

Only some of the targets in port1.0 are executed in a sandbox. I do not think we have any units tests being executed in the sandbox.

There are also the ​tests with Portfiles. That would be the most realistic environment for this kind of test, so I would add it there.