tests: Don't use TEST_DRIVER_DIR in virTestCaptureProgramExecChild()

TEST_DRIVER_DIR is defined as "$(top_builddir)/src/.libs"; however,
as of commit bc6e206322, virDriverLoadModule() will search (the
absolute version of) that directory automatically, which means
passing it through the environment is no longer necessary.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>

tests/testutils.c

@@ -443,7 +443,6 @@ void virTestCaptureProgramExecChild(const char *const argv[],
     int stdinfd = -1;
     const char *const env[] = {
         "LANG=C",
-        "LIBVIRT_DRIVER_DIR=" TEST_DRIVER_DIR,
         NULL
     };