fix var to enable git-svn tests with httpd

Upstream commit 3b072c577b (tests: replace test_tristate with "git
env--helper", 2019-06-21) semi-broke the git-svn tests which require
httpd.  This was subsequently fixed in upstream commit 6a20b62d7e
(t/lib-git-svn.sh: check GIT_TEST_SVN_HTTPD when running SVN HTTP tests,
2019-09-06).

The upstream fix also adjusted the variable name to follow the preferred
naming scheme, i.e. GIT_SVN_TEST_ -> GIT_TEST_SVN_.  Fix the variable in
%check to indicate that we want those tests to run.

We were still running the tests because we had all the necessary
dependencies.  But we want to ensure that we don't skip them
opportunistically if those dependencies ever change.

Update comment which suggest a method for (manually) checking such
variables in the test suite.

git.spec

@@ -826,11 +826,13 @@ export GIT_SKIP_TESTS
 export LANG=en_US.UTF-8
 
 # Explicitly enable tests which may be skipped opportunistically
-# (Check for variables set via test_tristate in the test suite)
-export GIT_SVN_TEST_HTTPD=true
+# Check for variables set via test_bool_env in the test suite:
+#   git grep 'test_bool_env GIT_' -- t/{lib-,t[0-9]}*.sh |
+#       sed -r 's/.* (GIT_[^ ]+) .*/\1/g' | sort -u
 export GIT_TEST_GIT_DAEMON=true
 export GIT_TEST_HTTPD=true
 export GIT_TEST_SVNSERVE=true
+export GIT_TEST_SVN_HTTPD=true
 
 # Create tmpdir for test output and update GIT_TEST_OPTS
 # Also update GIT-BUILD-OPTIONS to keep make from any needless rebuilding
@@ -998,6 +1000,7 @@ rmdir --ignore-fail-on-non-empty "$testdir"
 * Sun Jun 06 2021 Todd Zullinger <tmz@pobox.com> - 2.32.0-1
 - update to 2.32.0
 - add perl(File::Compare) BuildRequires
+- fix var to enable git-svn tests with httpd
 
 * Thu Jun 03 2021 Todd Zullinger <tmz@pobox.com> - 2.32.0-0.5.rc3
 - drop jgit on Fedora >= 35