Improve staticrootstest: add global data to library, add lib w/o GC_INIT

* tests/staticrootslib.c: Do not include stdio.h (not needed).
* tests/staticrootslib.c (root, root_nz): New static variables.
* tests/staticrootstest.c (root_nz): Likewise.
* tests/staticrootslib.c (libsrl_getpelem): New exported function
(exported as "libsrl_getpelem2" if STATICROOTSLIB2).
* tests/staticrootslib.c (libsrl_mktree, libsrl_init): Do not define
if STATICROOTSLIB2.
* tests/staticrootslib.c (libsrl_init): Do not call GC_INIT if
STATICROOTSLIB_INIT_IN_MAIN defined.
* tests/staticrootstest.c (libsrl_getpelem, libsrl_getpelem2): Declare
imported function.
* tests/staticrootstest.c (main): Call GC_INIT if
STATICROOTSLIB_INIT_IN_MAIN defined; set *libsrl_getpelem2() (only if
STATICROOTSLIB2 defined), *libsrl_getpelem() and root_nz[] elements
(similar to that of root[]).
Adjust build scripts to enable additional test library in staticrootstest
(The 2nd test library contains global data too but has no GC_INIT call.)

* tests/tests.am (staticrootstest_CFLAGS,
libstaticrootslib2_test_la_CFLAGS): New definition (to test with
libsrl_getpelem2() enabled).
* tests/tests.am (staticrootstest_LDADD, check_LTLIBRARIES): Add
libstaticrootslib2_test.la.
* tests/tests.am (libstaticrootslib2_test_la_SOURCES,
libstaticrootslib2_test_la_LIBADD,
libstaticrootslib2_test_la_LDFLAGS): New definition (similar to that
of libstaticrootslib_test_la_X).