perf test: Simplify for_each_test() to avoid tripping on -Werror=array-bounds

When cross building on debian to the mips 32-bit arch we get these
warnings:

In function '__cmd_test',
inlined from 'cmd_test' at tests/builtin-test.c:561:9:
tests/builtin-test.c:260:66: error: array subscript 1 is outside array bounds of 'struct test_suite *[1]' [-Werror=array-bounds]
260 | for (k = 0, t = tests[j][k]; tests[j][k]; k++, t = tests[j][k])
| ^
tests/builtin-test.c:369:9: note: in expansion of macro 'for_each_test'
369 | for_each_test(j, k, t) {
| ^~~~~~~~~~~~~
tests/builtin-test.c: In function 'cmd_test':
tests/builtin-test.c:36:27: note: at offset 4 into object 'arch_tests' of size 4
36 | struct test_suite *__weak arch_tests[] = {
| ^~~~~~~~~~
cc1: all warnings being treated as errors

Switch to using a while(!sentinel) for the second level of the 'tests'
array to avoid that compiler complaint.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>