Modify generator tracking to allow GENERATEs between SECTIONs

This means that code such as

```cpp
TEST_CASE() {
    SECTION("first") { SUCCEED(); }
    auto _ = GENERATE(1, 2);
    SECTION("second") { SUCCEED(); }
}
```

will run and report 3 assertions, 1 from section "first" and 2
from section "second". This also applies for greater and potentially
more confusing nesting, but fundamentally it is up to the user to
avoid overly complex and confusing nestings, just as with `SECTION`s.

The old behaviour of `GENERATE` as first thing in a `TEST_CASE`,
`GENERATE` not followed by a `SECTION`, etc etc should be unchanged.

Closes #1938

src/catch2/internal/catch_test_case_tracker.hpp

@@ -55,6 +55,7 @@ namespace TestCaseTracking {
         virtual bool isSuccessfullyCompleted() const = 0;
         virtual bool isOpen() const = 0; // Started but not complete
         virtual bool hasChildren() const = 0;
+        virtual bool hasStarted() const = 0;
 
         virtual ITracker& parent() = 0;
 
@@ -121,7 +122,9 @@ namespace TestCaseTracking {
         bool isSuccessfullyCompleted() const override;
         bool isOpen() const override;
         bool hasChildren() const override;
-
+        bool hasStarted() const override {
+            return m_runState != NotStarted;
+        }
 
         void addChild( ITrackerPtr const& child ) override;