test: fix parser to prefer binop over unop, as coreutils does.

test: fix parser to prefer binop over unop, as coreutils does.
 remove bogus workaround in main(). rename atrocious variables/functions.
 much expand testsuite.
libbb: fix --help to not affect "test --help"

function                                             old     new   delta
run_applet_no_and_exit                               421     440     +19
nexpr                                                817     825      +8
static.no_op                                           -       6      +6
test_main                                            397     257    -140
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(add/remove: 2/1 grow/shrink: 2/1 up/down: 104/-211)         Total: -107 bytes