Restore unit tests

Commit dbd42bb (Cleaning up zig build output) broke the unit test.

Always use exit code 2, instead of 0.  This is the exit code used by the
build runner to notify the compiler to not report any further
diagnostics.

Move the Ziglings logo from the `build` function scope to the global
scope, and make it public so that tests.zig can use it to find the
number of lines to skip, instead of using an hard coded value.

Fixes #295

test/tests.zig

@@ -175,7 +175,7 @@ pub fn addCliTests(b: *std.Build, exercises: []const Exercise) *Step {
         const cmd = b.addSystemCommand(&.{ b.zig_exe, "build", "-Dn=1" });
         const expect = exercises[0].hint orelse "";
         cmd.setName("zig build -Dn=1");
-        cmd.expectExitCode(1);
+        cmd.expectExitCode(2);
         cmd.addCheck(.{ .expect_stderr_match = expect });
 
         cmd.step.dependOn(case_step);
@@ -282,10 +282,11 @@ const CheckStep = struct {
         for (exercises) |ex| {
             if (ex.number() == 1 and self.has_logo) {
                 // Skip the logo.
+                const nlines = mem.count(u8, root.logo, "\n");
                 var buf: [80]u8 = undefined;
 
                 var lineno: usize = 0;
-                while (lineno < 8) : (lineno += 1) {
+                while (lineno < nlines) : (lineno += 1) {
                     _ = try readLine(stderr, &buf);
                 }
             }