link-tests: extract values into variables

We can then collect multiple variables (currently assumed always
in global scope) and run a comparison with some very basic
arithmetic on the values.

test/link/macho/entry/build.zig

@@ -14,8 +14,17 @@ pub fn build(b: *Builder) void {
     exe.entry_symbol_name = "_non_main";
 
     const check_exe = exe.checkMachO();
+
+    check_exe.check("segname __TEXT");
+    check_exe.checkNextExtract("vmaddr {vmaddr}");
+
     check_exe.check("cmd MAIN");
-    check_exe.checkNext("entryoff {x}");
+    check_exe.checkNextExtract("entryoff {entryoff}");
+
+    check_exe.checkInSymtab();
+    check_exe.checkNextExtract("_non_main {n_value}");
+
+    check_exe.checkCompare("{vmaddr entryoff +}", .{ .varr = "n_value" });
 
     test_step.dependOn(&check_exe.step);