std.debug.assert: remove special case for test builds

Previously, std.debug.assert would `@panic` in test builds,
if the assertion failed. Now, it's always `unreachable`.

This makes release mode test builds more accurately test
the actual code that will be run.

However this requires tests to call `std.testing.expect`
rather than `std.debug.assert` to make sure output is correct.

Here is the explanation of when to use either one, copied from
the assert doc comments:

Inside a test block, it is best to use the `std.testing` module
rather than assert, because assert may not detect a test failure
in ReleaseFast and ReleaseSafe mode. Outside of a test block, assert
is the correct function to use.

closes #1304

std/atomic/stack.zig

@@ -1,6 +1,7 @@
 const assert = std.debug.assert;
 const builtin = @import("builtin");
 const AtomicOrder = builtin.AtomicOrder;
+const expect = std.testing.expect;
 
 /// Many reader, many writer, non-allocating, thread-safe
 /// Uses a spinlock to protect push() and pop()
@@ -108,14 +109,14 @@ test "std.atomic.stack" {
         {
             var i: usize = 0;
             while (i < put_thread_count) : (i += 1) {
-                std.debug.assertOrPanic(startPuts(&context) == 0);
+                expect(startPuts(&context) == 0);
             }
         }
         context.puts_done = 1;
         {
             var i: usize = 0;
             while (i < put_thread_count) : (i += 1) {
-                std.debug.assertOrPanic(startGets(&context) == 0);
+                expect(startGets(&context) == 0);
             }
         }
     } else {