Merge branch 'jk/test-send-sh-x-trace-elsewhere' into maint

Running tests with '-x' option to trace the individual command
executions is a useful way to debug test scripts, but some tests
that capture the standard error stream and check what the command
said can be broken with the trace output mixed in.  When running
our tests under "bash", however, we can redirect the trace output
to another file descriptor to keep the standard error of programs
being tested intact.

* jk/test-send-sh-x-trace-elsewhere:
  test-lib: set BASH_XTRACEFD automatically

diff --cc t/README
Simple merge

diff --cc t/test-lib.sh
Simple merge