Format shell scripts with shfmt.

All changes in this patch were performed automatically with `shfmt` with
configuration flags specified in `.pre-commit-config.yaml`.

In addition to fixing whitespace the roundtrip through shfmt's AST also
transforms command substitutions

    `cmd`
    # becomes
    $(cmd)

and some redirects

    >&2 echo "msg"
    # becomes
    echo >&2 "msg"

testing/btest/plugins/bifs-and-scripts.sh

@@ -60,4 +60,3 @@ EOF
 cat >activate.zeek <<EOF
 @load-plugin Demo::Foo
 EOF
-