[FR]: Add new GTest log level DEBUG

### Does the feature exist in the most recent commit?

No

### Why do we need this feature?

Sometimes it is needed to have debug output in the tests and later to disable it, it's easier to not comment it out, just change log level to higher.

### Describe the proposal.

In the `enum GTestLogSeverity` add new constant `GTEST_DEBUG` (as the first one in the enum definition) and change code whenever its adding affects (before the first one in the enum was GTEST_DEBUG). To avoid possible breakage of existing code, which expects GTEST_INFO to have value `0`, maybe it will be more wise to define it as `GTEST_DEBUG = -1`.

### Is the feature specific to an operating system, compiler, or build system version?

No

Start by locating the enum GTestLogSeverity and every use affected by adding GTEST_DEBUG. Review how GTEST_INFO is currently valued and how log severity is compared or selected. Done means the new level is supported without breaking existing code that expects GTEST_INFO to be 0, with the relevant test behavior covered.

Written by the indexing model from the issue text.