It used to always try to divide by zero like:

    FAIL 1/0 tests failed

Some tests in btest uses long src for btest and it is harmful to
check the results. This patch introducing the limitation how many
lines of code is shown on failure.

GC benchmarks will spend long time with assertions so we need
to respect `RUBY_TEST_SUBPROCESS_TIMEOUT_SCALE` environment variable.

@nobu pointed out that now `RUBY_TEST_TIMEOUT_SCALE` is primary
(and `RUBY_TEST_SUBPROCESS_TIMEOUT_SCALE` was obsolete so check both
and will remove it later.