tests: simplify "missing PREREQ" message

When a test has no prerequisites satisfied (the usual case), instead of "missing THING of THING", just say "missing THING". This does not affect the output when a test is skipped due to a missing prerequisites if another prerequisite is satisfied. For example: instead of ok 8 # skip notes work (missing EXPENSIVE of EXPENSIVE) ok 9 # skip notes timing with /usr/bin/time (missing EXPENSIVE of USR_BIN_TIME,EXPENSIVE) write ok 8 # skip notes work (missing EXPENSIVE) ok 9 # skip notes timing with /usr/bin/time (missing EXPENSIVE of USR_BIN_TIME,EXPENSIVE) Cc: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>