utils.encoding.auto_decode() was broken when decoding Big Endian BOM
byte-strings on Little Endian or vice versa.

The TestEncoding.test_auto_decode_utf_16_le test was failing on Big Endian
systems, such as Fedora's s390x builders. A similar test, but with BE BOM
test_auto_decode_utf_16_be was added in order to reproduce this on a Little
Endian system (which is much easier to come by).

A regression test was added to check that all listed encodings in
utils.encoding.BOMS are valid.

Fixes https://github.com/pypa/pip/issues/6054

last reference removed in 52d87f2e

Why? Eases post-facto analysis of time spent in different phases of pip operation.

Historical note:
https://github.com/pypa/pip/commit/767d11e49cb916e2d4637421d524efcb8d02ae8d#diff-b670e3b192038c9ffe810c1a12c0c51fL219
made it so that pip invocations emit zero timestamp information to the log
file. Prior to that each pip invocation's start time was written out (search
that commit's diff for [strftime]).

Result: https://gist.github.com/fischman/f570886219de5c64a3b695300195c70a

Resolves https://github.com/pypa/pip/issues/6141

Also... make the tests actually pass.

So it is actually picked up correctly by Pytest.