Reorganize autotest files, factorize for parallel execution.

Reorganize autotest files, factorize for parallel execution.

* lib/autotest/general.m4 (AS_MESSAGE_LOG_FD): Move definition
earlier in the file.
(AT_INIT): Create line number cache in
$at_suite_dir/at-source-lines.
<at_helper_dir>: New directory at-groups below $at_suite_dir.
Add comment explaining the new directory structure.
(at_func_group_prepare, at_func_group_postprocess): New shell
functions to factorize per-test group work.  Keep the actual
test execution outside of a shell function in order to avoid
zsh 4.x exit status bugs.
<at_check_line_file, at_status_file, at_stdout, at_stder1>
<at_stderr, at_test_source>: Turn these into per-group files
below $at_helper_dir.  Also store test results there in files
named pass, fail, xpass, xfail, skip.  Let the parent collect
results from $at_helper_dir.  Adjust summary statistics
computation and result output.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>