LibTest: Change #define-d constants into constexpr and a runtime flag

MAX_GENERATED_VALUES_PER_TEST is now the --randomized_runs flag:
$ ./Build/lagom/bin/TestGenerator --randomized_runs 1000

It's sometimes useful to try larger numbers for it instead of the
default of 100.

MAX_GEN_ATTEMPTS_PER_VALUE is now a constexpr. It's not usually needed
to tweak this value; we can recompile with a different value on the rare
occasion.

Userland/Libraries/LibTest/Macros.h

@@ -30,6 +30,8 @@ void set_randomness_source(Randomized::RandomnessSource);
 bool is_reporting_enabled();
 void enable_reporting();
 void disable_reporting();
+
+u64 randomized_runs();
 }
 
 #define EXPECT_EQ(a, b)                                                                                       \