gtestutils: print the TAP test plan first, not last

TAP allows you to print the "test plan" (ie, the expected number of
tests" either at the start or the end of the test program, but if you
put it at the end, and the program crashes, automake will complain
"missing test plan", which is confusing to users (particularly since
it prints that *before* it prints that the test program crashed,
suggesting that somehow the lack of test plan was responsible for the
crash or something, rather than vice versa).

Anyway, change it to count the tests ahead of time, and print the test
plan first. Keeping this simple requires disallowing the '-p', '-s',
and '--GTestSkipCount' options when using '--tap' (although we were
already printing the wrong number in the --GTestSkipCount case
anyway).

https://bugzilla.gnome.org/show_bug.cgi?id=754284