tests: do not require Git to be built when testing an installed Git

We really only need the test helpers in that case, but that is not what
we test for. So let's skip the test for now when we know that we want to
test an installed Git.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/test-lib.sh

@@ -62,7 +62,7 @@ PATH="$GIT_BUILD_DIR/$MSVC_DEPS/bin:$PATH"
 
 ################################################################
 # It appears that people try to run tests without building...
-"$GIT_BUILD_DIR/git$X" >/dev/null
+test -n "$GIT_TEST_INSTALLED" || "$GIT_BUILD_DIR/git$X" >/dev/null ||
 if test $? != 1
 then
 	echo >&2 'error: you do not seem to have built git yet.'