t3000 (ls-files -o): modernize style

t3000 (ls-files -o): modernize style

This script is part of the second batch of tests, from the same day
the test infrastructure was added to git.  Update it to use a more
modern style in the spirit of v1.6.4-rc0~45^2~2 (2009-05-22).
In particular:

 - Put setup code inside test assertions, to avoid unexpected
   breakages and avoid stray output without -v (as t/README
   recommends); and

 - Put the test title on the same line as the "test_expect_success",
   and end the line with a single-quote to begin the body of the test
   which is one multi-line string.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>