Allow "" as specification of an empty line for stdin #598
Description
TESTed guarantees POSIX-standard complience, by always making sure each line (including the last line) ends with a newline. If a trailing newline isn't specified in the test suite, TESTed automatically appends one (to avoid test suites that are clutterd with trailing newlines).
However, the empty string "" is treated differently; If we specify stdin: "", this is treated as "no lines on stdin" (which is also what we get is the stdin attribute is missing), where it should better be interpreted as "\n" to follow what TESTed does with all other strings. Now, the only way to specify an "empty line" as input (and probably also output) is by specifying "\n".