t/lib-git-svn.sh: check GIT_TEST_SVN_HTTPD when running SVN HTTP tests

t/lib-git-svn.sh: check GIT_TEST_SVN_HTTPD when running SVN HTTP tests

Once upon a time the GIT_SVN_TEST_HTTPD environment variable needed to
be set to enable SVN HTTP tests [1].

Then 3b072c577b (tests: replace test_tristate with "git env--helper",
2019-06-21) came along, and attempted to turn GIT_SVN_TEST_HTTPD into
a bool, but while doing so it mistyped the variable name, and started
to check GIT_TEST_HTTPD instead.  Consequently, if someone explicitly
set GIT_TEST_HTTPD to true and has only the general 'git-svn'
dependencies installed but not the Subversion server modules for
Apache (libapache2-mod-svn), then a couple of 'git-svn' tests fail,
because they can't start httpd due to the missing module.

We could simply fix this by checking the GIT_SVN_TEST_HTTPD
variablewith 'git env--helper', but notice that the name of this
variable doesn't conform to our usual GIT_TEST_* convention.

So let's check the GIT_TEST_SVN_HTTPD instead.

[1] a8a5d25118 (git svn: migrate tests to use lib-httpd, 2016-07-23)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>