[AIX] Split lit test shtest-format into two separate tests and add AIX as UNSUPPORTED...

[AIX] Split lit test shtest-format into two separate tests and add AIX as UNSUPPORTED for shtest-format-argv0

Summary: lit test `shtest-format.py` fails on AIX  because one of the subtest of shtest-format requires the tool `[` to be installed under the system PATH. For AIX, `[` is only available as a shell builtin and does not present as an executable file under PATH. Hence, split the original shtest-format into two separate test files and added AIX as UNSUPPORTED for the test using `[` .

Reviewers: daltenty, hubert.reinterpretcast

Reviewed By: hubert.reinterpretcast

Subscribers: delcypher, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82100