tests: replace test_tristate with "git env--helper"

tests: replace test_tristate with "git env--helper"

The test_tristate helper introduced in 83d842dc8c ("tests: turn on
network daemon tests by default", 2014-02-10) can now be better
implemented with "git env--helper" to give the variables in question
the standard boolean behavior.

The reason for the "tristate" was to have all of false/true/auto,
where "auto" meant either "false" or "true" depending on what the
fallback was. With the --default option to "git env--helper" we can
simply have e.g. GIT_TEST_HTTPD where we know if it's true because the
user asked explicitly ("true"), or true implicitly ("auto").

This breaks backwards compatibility for explicitly setting "auto" for
these variables, but I don't think anyone cares. That was always
intended to be internal.

This means the test_normalize_bool() code in test-lib-functions.sh
goes away in addition to test_tristate(). We still need the
test_skip_or_die() helper, but now it takes the variable name instead
of the value, and uses "git env--bool" to distinguish a default "true"
from an explicit "true" (in those "explicit true" cases we want to
fail the test in question).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>