Start getting all shell scripts passing shellcheck

Like with the formatter, we are blacklisting most files by default.

Do a few files to get us started, and get a sense of what this looks
like.

mk/common-test.sh

@@ -1,19 +1,24 @@
+# shellcheck shell=bash
+
 # Remove overall test dir (at most one of the two should match) and
 # remove file extension.
+
+# shellcheck disable=SC2154
 test_name=$(echo -n "$test" | sed \
     -e "s|^tests/unit/[^/]*/data/||" \
     -e "s|^tests/functional/||" \
     -e "s|\.sh$||" \
     )
 
+# shellcheck disable=SC2016
 TESTS_ENVIRONMENT=(
     "TEST_NAME=$test_name"
     'NIX_REMOTE='
     'PS4=+(${BASH_SOURCE[0]-$0}:$LINENO) '
 )
 
-: ${BASH:=/usr/bin/env bash}
+read -r -a bash <<< "${BASH:-/usr/bin/env bash}"
 
 run () {
-   cd "$(dirname $1)" && env "${TESTS_ENVIRONMENT[@]}" $BASH -x -e -u -o pipefail $(basename $1)
+   cd "$(dirname "$1")" && env "${TESTS_ENVIRONMENT[@]}" "${bash[@]}" -x -e -u -o pipefail "$(basename "$1")"
 }