Give test messages unique timestamps

[David Bremner]

It turns out that having all messages with the same timestamp leads to
some nondeterministic behaviour of the test suite (see e.g.https://buildd.debian.org/status/fetch.php?pkg=notmuch&arch=i386&ver=0.17%7Erc3-1&stamp=1386412394) 

Decreasing timestamps are chosen to reduce the number of changes
needed to the test suite.  The first three patches are independent of
this choice.


 test/emacs           | 12 +++++++-----
 test/emacs-show      | 10 ++++++----
 test/encoding        |  8 ++++----
 test/excludes        |  4 ++--
 test/maildir-sync    | 15 +++++----------
 test/missing-headers |  8 +++++---
 test/multipart       |  2 +-
 test/raw             |  8 ++++----
 test/test-lib.sh     | 15 ++++++++++++---
 9 files changed, 46 insertions(+), 36 deletions(-)