test: prepare test-lib.sh for possible test system debug session

test: prepare test-lib.sh for possible test system debug session

When something in tests fails one possibility to test is to run
the test script as `bash -x TXXX-testname.sh`. As stderr (fd 2) was
redirected to separate file during test execution also this set -x
(xtrace) output would also go there.
test-lib.sh saves the stderr to fd 7 from where it can be restored,
and bash has BASH_XTRACEFD variable, which is now given the same value
7, making bash to output all xtrade information (consistently) there.

This lib file used to save fd's 1 & 2 to 6 & 7 (respectively) in
test_begin_subtest(), but as those needs to be set *before* XTRACEFD
variable is set those are now saved at the beginning of the lib (once).
This is safe and simple thing to do.
To make xtrace output more verbose PS4 variable was set to contain the
source file, line number and if execution is in function, that function
name. Setting this variable has no effect when not xtracing.

As it is known that fd 6 is redirected stdout, printing status can now
use that fd, instead of saving stdout to fd 5 and use it.