tests: Run global cleanup at end of tests

Make sure to run curl_global_cleanup() when shutting down the test
suite to release any resources allocated in the SSL setup. This is
clearly visible when running tests with PolarSSL where the thread
lock calloc() memory which isn't released when not running cleanup.
Below is an excerpt from the autobuild logs:

  ==12368== 96 bytes in 1 blocks are possibly lost in loss record 1 of 2
  ==12368== at 0x4837B65: calloc (vg_replace_malloc.c:752)
  ==12368== by 0x11A76E: curl_dbg_calloc (memdebug.c:205)
  ==12368== by 0x145CDF: Curl_polarsslthreadlock_thread_setup
                         (polarssl_threadlock.c:54)
  ==12368== by 0x145B37: Curl_polarssl_init (polarssl.c:865)
  ==12368== by 0x14129D: Curl_ssl_init (vtls.c:171)
  ==12368== by 0x118B4C: global_init (easy.c:158)
  ==12368== by 0x118BF5: curl_global_init (easy.c:221)
  ==12368== by 0x118D0B: curl_easy_init (easy.c:299)
  ==12368== by 0x114E96: test (lib1906.c:32)
  ==12368== by 0x115495: main (first.c:174)

Closes #3783
Reviewed-by: Marcel Raad <Marcel.Raad@teamviewer.com>
Reviewed-by: Daniel Stenberg <daniel@haxx.se>