efi_selftest: add tests for setvariableRT

Since we support SetVariableRT now add the relevant tests

- Search for the RTStorageVolatile and VarToFile variables after EBS
- Try to update with invalid variales (BS, RT only)
- Try to write a variable bigger than our backend storage
- Write a variable that fits and check VarToFile has been updated
  correclty
- Append to the variable and check VarToFile changes
- Try to delete VarToFile which is write protected
- Try to add/delete runtime variables
- Verify VarToFile contains a valid file format

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>