tests: turn on network daemon tests by default

tests: turn on network daemon tests by default

We do not run the httpd nor git-daemon tests by default, as
they are rather heavyweight and require network access
(albeit over localhost). However, it would be nice if more
pepole ran them, for two reasons:

  1. We would get more test coverage on more systems.

  2. The point of the test suite is to find regressions. It
     is very easy to change some of the underlying code and
     break the httpd code without realizing you are even
     affecting it. Running the httpd tests helps find these
     problems sooner (ideally before the patches even hit
     the list).

We still want to leave an "out", though, for people who really do
not want to run them. For that reason, the GIT_TEST_HTTPD and
GIT_TEST_GIT_DAEMON variables are now tri-state booleans
(true/false/auto), so you can say GIT_TEST_HTTPD=false to turn the
tests back off.  To support those who want a stable single way to
disable these tests across versions of Git before and after this
change, an empty string explicitly set to these variables is also
taken as "false", so the behaviour changes only for those who:

  a. did not express any preference by leaving these variables
     unset.  They did not test these features before, but now they
     do; or

  b. did express that they want to test these features by setting
     GIT_TEST_FEATURE=false (or any equivalent other ways to tell
     "false" to Git, e.g. "0"), which has been a valid but funny way
     to say that they do want to test the feature only because we
     used to interpret any non-empty string to mean "yes please
     test".  They no longer test that feature.

In addition, we are forgiving of common setup failures (e.g., you do
not have apache installed, or have an old version) when the
tri-state is "auto" (or unset), but report an error when it is
"true". This makes "auto" a sane default, as we should not cause
failures on setups where the tests cannot run. But it allows people
who use "true" to catch regressions in their system (e.g., they
uninstalled apache, but were expecting their automated test runs to
test git-httpd, and would want to be notified).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>