Workaround AppleClang bug by renaming TestHasher constructor argument

As far as I understand the standard, if there is a function called
`rng` in the global namespace, and a function argument called `rng`,
then the argument should shadow the function. This then means that
uses of `rng` inside the function should refer to the argument.

This is not the case for AppleClang 12.0.0. Luckily the workaround
is simple enough; just rename the argument. Given that the function
is 3 lines and uncomplicated, the change of the name doesn't really
affect readability.

Still, WTF AppleClang?

Closes #2030

src/catch2/internal/catch_test_case_registry_impl.cpp

@@ -23,10 +23,10 @@ namespace Catch {
 
 namespace {
     struct HashTest {
-        explicit HashTest(SimplePcg32& rng) {
+        explicit HashTest(SimplePcg32& rng_inst) {
-            basis = rng();
+            basis = rng_inst();
             basis <<= 32;
-            basis |= rng();
+            basis |= rng_inst();
         }
 
         uint64_t basis;