Allow group test functions in Value-Parametrized Tests

Let's say I have test data like this (looks like in #2566 ? No accident!):

```cpp
struct test_data
{
foo f;
bool bar;
} tests[] = {
{ foo(), true },
{ foo(), false },
// ...
};

class X : public testing::TestWIthParam{}

TEST_P(X, first) {}
TEST_P(X, second) {}
```

For the most part, this is exactly what I want. But let's say my `foo` type should behave like a value type, including hashing to sufficiently unique values.

I can always define a standalone test case to do that:

```cpp
TEST(Z, unique_hashes)
{
std::set hashes;
std::hash hasher;

for (size_t i = 0 ; i < sizeof(tests) / sizeof(test_data) ; ++i) {
hashes.insert(hasher(tests[i].f);
}

// The expecation is that the size of the set is the same as the size of the test data
ASSERT_EQ(sizeof(tests) / sizeof(test_data), hashes.size());
}
```

It works just fine - but the `Z` test is reported as its own unique group when really it belongs to tests on the same test data. I'd like to see something like this:

```cpp
TEST_PG(X, unique_hashes)
{
std::set hashes;
std::hash hasher;

for (auto item : GetValues()) {
hashes.insert(hasher(item.f);
}

// The expecation is that the size of the set is the same as the size of the test data
ASSERT_EQ(GetValues().size(), hashes.size());
}
```

Which then gets instantiated along with the rest of the suite. Except of course only once, not once for each value.

Start by tracing how TEST_P registers value-parameterized tests and how the supplied test_data array is instantiated. Investigate how a grouped function such as the proposed TEST_PG could access all values and be reported with the same suite; done means it runs once per suite rather than once per value while remaining grouped with the other tests.

Written by the indexing model from the issue text.