lit/TestingConfig.py: Add the environment variable PRINTF_EXPONENT_DIGITS as workarou...

lit/TestingConfig.py: Add the environment variable PRINTF_EXPONENT_DIGITS as workaround [PR6745] for mingw's tests. Danil Malyshev suggested this.

FIXME: It does not improve MSVC's issue.

[Danil Malyshev] Defining PRINTF_EXPONENT_DIGITS env is the suggested way to make MinGW ANSI/POSIX compatible. This is not only about the case we are discussing, but in general, I'd like to have explicitly defined compatibility mode for all the tests running on MinGW.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125725 91177308-0d34-0410-b5e6-96231b3b80d8