The check unit test framework isn't particularly suited to having ck_assert*
calls in helper functions. A failed assertion in a helper function or the
litest framework merely gives us a the failed line in litest.c.
which doesn't tell us which test actually failed.

Add a backtracing facility with litest_backtrace(). And since this requires
wrapping all ck_assert macros with litest_assert() this patch ended up
replacing/duplicating a bunch of ck_assert_* bits. So rather than
ck_assert_int_eq() we now use litest_assert_int_eq(), etc. in the litest
framework itself.

The int comparison macros are more type-safe than ck_assert()'s macros which
just cast all the ints to intmax_t.

Backtrace is spewed to stderr, which is good enough for debugging. Example
backtrace:

	FAILED COMPARISON: status != expected
	Resolved to: 0 != 0
	in disable_button_scrolling() (pointer.c:115)

	Backtrace:
	0: ./test/test-pointer (litest_fail_comparison_int+0xab) [0x40973b]
	1: ./test/test-pointer (disable_button_scrolling+0x174) [0x40421b]
	2: ./test/test-pointer (middlebutton_middleclick+0x40) [0x40829c]
	3: /lib64/libcheck.so.0 (srunner_run+0x7f5) [0x7f0e8a277025]
	4: ./test/test-pointer (litest_run+0x107) [0x40a42b]
	5: ./test/test-pointer (main+0x2fa) [0x4090e7]
	6: /lib64/libc.so.6 (__libc_start_main+0xf0) [0x7f0e88f5e790]
	7: ./test/test-pointer (_start+0x29) [0x403ce9]
	8: ? (?+0x29) [0x29]

litest_backtrace() itself is copied from xserver/os/backtrace.c which git
blame attributes to Marcin.

CC:  Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>