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

include/internal/catch_test_case_registry_impl.cpp

@@ -22,10 +22,10 @@ namespace Catch {
 
     namespace {
         struct TestHasher {
-            explicit TestHasher(Catch::SimplePcg32& rng) {
-                basis = rng();
+            explicit TestHasher(Catch::SimplePcg32& rng_instance) {
+                basis = rng_instance();
                 basis <<= 32;
-                basis |= rng();
+                basis |= rng_instance();
             }
 
             uint64_t basis;