[#1034] Fix MUPIP EXTEND to accept case-insensitive region names

After a thorough investigation, it was determined that the only MUPIP operation that did not support case-insensitive region names was MUPIP EXTEND. The investigation involved consulting the YDB documentation on MUPIP, and running every operation that involves region names as a possible option, in the gdb debugger, to determine how it handles them.

After that, the test suite was searched for all tests that involved the above mentioned operations that involve region names, to ensure that every one of them had at least one test that used mixed-case region names. If an operation was found that did not have such a test case, an existing one was modified. In every case, it was found that there was always more than one test case using a region name, so in the end, no tests lost any coverage, and coverage for case-insensitive names was added.

Then, the test suite for every applicable test was run and that confirmed that no other MUPIP operation was lacking case-insensitive region name support.