test: copyright information updates

Files in test directories had only copyright of a single individual,
of which code was adapted here as a base of the test system.
Since then many Notmuch Developers have contributed to the test
system, which is now acknowledged with a constant string in some
of the test files.

The README file in test directory instructed new files contain a
copyright notice, but that has never been done (and it is also not
needed). To simplify things a bit (and lessen confusion) this
instruction is now removed.

As a side enchangement, all of the 3 entries in the whole source
tree cd'ing to `dirname` of "$0" now uses syntax cd "$(dirname "$0")".
This makes these particular lines work when current working directory
is e.g. /c/Program Files/notmuch/test/.
(Probably it would fail elsewhere, though.)