Rename the json_view test helpers that clash in the unity build

unit-json_view_builder.cpp has its own document generator besides
json_view_test::generator, and unit-json_view_edit.cpp and
unit-json_view_image.cpp define rng() and bits() like unit-json_view.cpp,
each in an anonymous namespace. In one batch these are ambiguous or
redefined, so give the file-local ones their own names.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_builder.cpp

@@ -173,7 +173,7 @@ void check_string(const std::string& content)
 }
 
 // a small deterministic generator of documents
-struct generator
+struct builder_generator
 {
     std::mt19937 rng{5295}; // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed)
 
@@ -407,7 +407,7 @@ TEST_CASE("json_view builder")
 
     SECTION("generated documents and damaged copies")
     {
-        generator g;
+        builder_generator g;
         for (int i = 0; i < 3000; ++i)
         {
             std::string text;