tests/oauth2: Fix 'token' leaks
tests/oauth2: Fix 'token' leaks

The string returned from oauth2_proxy_extract_access_token() must be
freed. This fixes:

==16342== 27 bytes in 1 blocks are definitely lost in loss record 125 of 278
==16342==    at 0x4C2BBAD: malloc (vg_replace_malloc.c:299)
==16342==    by 0x5F5CE58: g_malloc (gmem.c:94)
==16342==    by 0x5F75917: g_strndup (gstrfuncs.c:425)
==16342==    by 0x56D9BA4: soup_uri_decoded_copy (soup-uri.c:761)
==16342==    by 0x4E48C0B: oauth2_proxy_extract_access_token (oauth2-proxy.c:392)
==16342==    by 0x400A25: test_access_token_simple (oauth2.c:44)
==16342==    by 0x5F7C983: test_case_run (gtestutils.c:2158)
==16342==    by 0x5F7C983: g_test_run_suite_internal (gtestutils.c:2241)
==16342==    by 0x5F7CB4E: g_test_run_suite_internal (gtestutils.c:2253)
==16342==    by 0x5F7CD5D: g_test_run_suite (gtestutils.c:2328)
==16342==    by 0x5F7CD80: g_test_run (gtestutils.c:1596)
==16342==    by 0x400B64: main (oauth2.c:65)
==16342==
==16342== 27 bytes in 1 blocks are definitely lost in loss record 126 of 278
==16342==    at 0x4C2BBAD: malloc (vg_replace_malloc.c:299)
==16342==    by 0x5F5CE58: g_malloc (gmem.c:94)
==16342==    by 0x5F75917: g_strndup (gstrfuncs.c:425)
==16342==    by 0x56D9BA4: soup_uri_decoded_copy (soup-uri.c:761)
==16342==    by 0x4E48C0B: oauth2_proxy_extract_access_token (oauth2-proxy.c:392)
==16342==    by 0x400A97: test_url_encoding_access_token (oauth2.c:51)
==16342==    by 0x5F7C983: test_case_run (gtestutils.c:2158)
==16342==    by 0x5F7C983: g_test_run_suite_internal (gtestutils.c:2241)
==16342==    by 0x5F7CB4E: g_test_run_suite_internal (gtestutils.c:2253)
==16342==    by 0x5F7CD5D: g_test_run_suite (gtestutils.c:2328)
==16342==    by 0x5F7CD80: g_test_run (gtestutils.c:1596)
==16342==    by 0x400B64: main (oauth2.c:65)

GNOME/librest,GNOME/librest,GNOME/librest