tests: simplify "missing PREREQ" message

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>