test: print assertion backtraces

[camshaft]

Problem:

In the test assertions we only print the stacktrace from s2n if it encountered any errors. This doesn't apply to EXPECT_TRUE, `EXPECT_EQ, etc. When using these macros inside of functions, it makes it difficult to know exactly why the assertion failed.

Solution:

The FAIL_MSG_PRINT should be updated to include the stack trace of the actual assertion. We can use the same backtrace function as the library:

s2n-tls/error/s2n_errno.c

Lines 390 to 391 in 1ff0aa8

	tl_stacktrace.trace_size = backtrace(array, MAX_BACKTRACE_DEPTH);
	tl_stacktrace.trace = backtrace_symbols(array, tl_stacktrace.trace_size);