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/zig/parser_test.zig

@@ -1940,7 +1940,7 @@ fn testTransform(source: []const u8, expected_source: []const u8) !void {
             warn("std.zig.render returned {} instead of {}\n", anything_changed, changes_expected);
             return error.TestFailed;
         }
-        std.debug.assert(anything_changed == changes_expected);
+        std.testing.expect(anything_changed == changes_expected);
         failing_allocator.allocator.free(result_source);
         break :x failing_allocator.index;
     };