tests/test-lrucachedict.py.out

debugcommands: introduce actions to perform deterministic reads

"readavailable" is useful as a debugging device to see what data is
available on a pipe. But the mechanism isn't deterministic because
what's available on a pipe is highly conditional on timing, system
load, OS behavior, etc. This makes it not suitable for tests.

We introduce "ereadline," "read," and "eread" for performing
deterministic I/O operations (at least on blocking file descriptors).

We stop short of converting existing consumers of "readavailable"
in tests because we're working out race conditions and deadlocks
on Windows. But the goal is to eventually move tests away from
"readavailable" to these new APIs.

Differential Revision: https://phab.mercurial-scm.org/D2720

'a' in d: True
d['a']: va
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'a' in d: False
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'e' in d: True
d['e']: ve
'b' in d: True
d['b']: vb2
'c' in d: True
d['c']: vc2
'd' in d: True
d['d']: vd
'e' in d: False
'f' in d: True
d['f']: vf
'b' in d: False
'c' in d: False
'd' in d: False
'e' in d: False
'f' in d: False
'a' in d: True
d['a']: 1
'b' in d: True
d['b']: 2

All of these should be present:
'a' in dc: True
dc['a']: va3
'b' in dc: True
dc['b']: vb3
'c' in dc: True
dc['c']: vc3
'd' in dc: True
dc['d']: vd3

All of these except 'a' should be present:
'a' in dc: False
'b' in dc: True
dc['b']: vb3
'c' in dc: True
dc['c']: vc3
'd' in dc: True
dc['d']: vd3
'e' in dc: True
dc['e']: ve3

These should be in reverse alphabetical order and read 'v?3':
d['d']: vd3
d['c']: vc3
d['b']: vb3
d['a']: va3