add XferTimeout TCR access functions to <l4/thread.h>
testbench: string: write basic tests about xfer timeouts

These tests are somewhat incomplete: first, the testing primitive
faulting_echo() should be proven to cause and not cause faults with a test in
the meta case. Second, the timeout tests should prod the send and receive phase
xfer faults separately. Third, there aren't nearly enough test points in there
that fail.
testbench: change TextReport TODO text to match condition

And change to-do to TODO.
testbench: string: add faulting_echo_test
testbench: string: fix echo_with_hole, add diag_faults() calls
testbench: add reset function to pg_stats.c
testbench: string: add send/recv unmap split to faulting_echo()

And test that it works. It does; breaking faulting_echo()'s unmaps causes a
different set of test points to fail, and no break goes unnoticed.
testbench: string: vary u_send, u_recv in immediate_xfer_timeout

This should be done with iterated testing, but until the test harness reports
it properly and all tests output streams are consistent between iterations,
the in-test for loop will suffice.
testbench: change colour for TODO test points' sigils
testbench: specify looped test length at declaration

Rather than the spot where it's added to a test case. If the test has a
different input range, it should be declared separately with a (so far
unnecessary) DERIVE_LOOP_TEST() macro.