testsuite: testsuite-mailstore - Return struct testsuite_mailstore_mail from testsuite_mailstore_open().

This is more flexible and needed in subsequent commit.

Attempting to pipeline any commands with the AUTHENTICATE command resulted
in crash.

Fixes:
Panic: file istream.c: line 317 (i_stream_read_memarea): assertion failed: (old_size <= _stream->pos - _stream->skip)

Those are currently exclusively system tools and we cannot control whether those
have memory leaks.

Even on a well configured system, the log may accumulate the occasion error.
Because of the rare nature, it is impossible to know if the error from
yesterday is really a year old error that doesn't matter anymore.

This is actually a secondary test. The command calling this should already have
checked this.

mailbox_get_last_error() should be used only for client-visible errors.
The internal errors should be used for logging.

client_destroy() already adds it.

It indicates that script execution exceeded a resource limit (currently only CPU
time limit).