Create helper macro for test name string

Nobody has claimed this yet.

There are cases where you need a unique string in test cases, for example creating the Sidre datastore. We should create a macro that returns a string of the test name. This will help with people accidentally using the same string to output files in different tests.

Something like:

```
#define GTEST_FULL_NAME() \
([]() -> std::string { \
const auto* info = \
::testing::UnitTest::GetInstance()->current_test_info(); \
return info \
? std::string(info->test_suite_name()) + "_" + info->name() \
: std::string{}; \
}())
```

It would return `SolidMechanics_AxisymmetricPressureInnerResultant` in this case.

_Originally posted by @white238 in https://github.com/llnl/smith/pull/1638#discussion_r3723021586_

Search the test utilities and Sidre datastore tests for existing helpers and cases that create output strings, then review the proposed GTEST_FULL_NAME() use of current_test_info(). Done means a reusable helper returns the test suite and test name joined as shown, avoids collisions across tests, and is covered by an appropriate test.

Written by the indexing model from the issue text.