Allow tagging by TestType in TEMPLATE_TEST_CASE

**Description**

In my own use case, I run tests of the kind:

```c++
TEMPLATE_TEST_CASE("MyTest", "[tag1][tag2]", float, double} {
[...]
}
```

where the tests are parameterised by the floating point precision.

I would love the ability to run just the float tests, or just the double tests, for example. However, at present it does not seem possible to tag by the `TestType`.

**Feature request:** allow for `TestType` to be used in the tag list, e.g.

```c++
TEMPLATE_TEST_CASE("MyTest", "[tag1][tag2][TestType]", float, double} {
[...]
}
```

thus allowing me to run: `./tests [float]` or `./tests [double]`.