LibTest: Add the GEN macro

Generators are callable as-is:

u32 my_int = Gen::unsigned_int(); // -> 1, 5, 8, 3, 2

But there is little visibility in the test fail message into what went
wrong. Showing what values were generated helps a lot, and that's what
this macro does:

GEN(my_int, Gen::unsigned_int());

expands into the above declaration and (crucially) a conditional
warnln() call looking like "my_int = {}". It will only run if error
reporting is enabled (see Test::can_report()), so it will only give the
final shrunk value instead of spamming the output with each generated
value.

Userland/Libraries/LibTest/TestCase.h

@@ -82,3 +82,11 @@ void set_suite_setup_function(Function<void()> setup);
     };                                                                                               \
     static struct __BENCHMARK_TYPE(x) __BENCHMARK_TYPE(x);                                           \
     static void __BENCHMARK_FUNC(x)()
+
+// This allows us to print the generated locals in the test after a failure is fully shrunk.
+#define GEN(identifier, value)                                        \
+    auto identifier = (value);                                        \
+    if (::Test::current_test_result() == ::Test::TestResult::Overrun) \
+        return;                                                       \
+    if (::Test::is_reporting_enabled())                               \
+    ::AK::warnln("{} = {}", #identifier, (identifier))