gtestutils: add g_test_trap_subprocess(), deprecate g_test_trap_fork()

g_test_trap_fork() doesn't work on Windows and is potentially flaky on
unix anyway given the fork-but-don't-exec. Replace it with
g_test_trap_subprocess(), which re-spawns the same program with
arguments telling it to run a specific (otherwise-ignored) test case.

Make the existing g_test_trap_fork() unit tests be unix-only (they
never passed on Windows anyway), and add a parallel set of
g_test_trap_subprocess() tests.

https://bugzilla.gnome.org/show_bug.cgi?id=679683

docs/reference/glib/glib-sections.txt

@@ -2888,6 +2888,7 @@ g_test_perf
 g_test_verbose
 g_test_undefined
 g_test_quiet
+g_test_subprocess
 g_test_run
 GTestFunc
 g_test_add_func
@@ -2895,6 +2896,7 @@ GTestDataFunc
 g_test_add_data_func
 g_test_add_data_func_full
 g_test_add
+g_test_case_exists
 
 g_test_fail
 g_test_message
@@ -2915,7 +2917,7 @@ g_test_expect_message
 g_test_assert_expected_messages
 
 GTestTrapFlags
-g_test_trap_fork
+g_test_trap_subprocess
 g_test_trap_has_passed
 g_test_trap_reached_timeout
 g_test_trap_assert_passed
@@ -2924,6 +2926,7 @@ g_test_trap_assert_stdout
 g_test_trap_assert_stdout_unmatched
 g_test_trap_assert_stderr
 g_test_trap_assert_stderr_unmatched
+g_test_trap_fork
 
 g_test_rand_bit
 g_test_rand_int