exercises: tests: remove comptime

[ee7]

Rationale:

• There is no more comptime in the tests that can potentially confuse a user. It was required in some of these exercises, e.g. when we wrote something like expected = 42.
• Test failure messages at comptime can sometimes be more confusing (see e.g. 7dc446e).
• The main downside is that, for some exercises, we no longer support the user returning a signed integer instead of an unsigned integer. But that's OK.

With this PR, the only remaining occurrence of comptime in this repo is in the binary-search example solution:

$ git rev-parse --short HEAD
06e7bce
$ git grep --break --heading comptime
exercises/practice/binary:search/.meta/example.zig
7:pub fn binarySearch(comptime T: type, target: T, buffer: ?[]const T) SearchError!usize {