[test] Add a .clang-format file for the shell test.
The API tests have a .clang-format file that disables formatting
altogether. While this is needed for some tests, it also leads to
inconsistency between test files. The shell tests suffer from a similar
problem: a test with a source-file extension (.c, .cpp) will get
formatted, potentially breaking up lines and leading to invalid RUN
Rather than completely disabling formatting here, I propose to not
enforce a line limit instead. That way tests will be consistent, but you
can still have long run commands (as is not uncommon in LLVM either) and
use breakpoints with patters that extend beyond 80 cols.
Differential revision: https://reviews.llvm.org/D69058