[flang][test-suite] Document need for NO_STOP_MESSAGE environment variable. NFC

When running the llvm-test-suite with flang, we get a lot of failures
because of the output of the `STOP` statement. We can workaround them by
setting `NO_STOP_MESSAGE=1` in the environment. This patch adds a few
words about it to the docs about the Fortran part of the llvm-test-suite.

See also https://reviews.llvm.org/D126360