autotest can not runs tests in parallel

[andrew-taylor]

Proposed approach - add these variables:

test_threads_max default value of number of cores
test_threads_n default value 1
tests_threads_files_copied default value list of supplied files

tests would be run in min(test_threads_max, test_threads_n) threads

if tests_threads_files_copied is non-empty run each test in a new directory,
after copying the specified files to the directory.