Speed up execution of subset of testsuite.
* lib/autotest/general.m4 (TEST_FUNCTIONS): New diversion.
(AT_INIT) <at_func_test>: New shell function.
(AT_INIT) <at_myself>: New variable, set to absolute $as_myself.
(AT_INIT) <at_test_source> New variable, names file that holds
current test function definition.
(AT_SETUP): Start the shell function at_func_test_#, into the
TEST_FUNCTIONS diversion.
(AT_CLEANUP): End the shell function. Simplify the TESTS
diversion to invoke the function.
Signed-off-by: Eric Blake <ebb9@byu.net>