Fix most shell hygiene issues found by shellcheck

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/tests/unicode-combine-fallback-assert.test

@@ -8,11 +8,12 @@
 #
 # It just sends the offending output to mosh and expects it to not die.
 
-. $(dirname $0)/e2e-test-subrs
+# shellcheck source=e2e-test-subrs
+. "$(dirname "$0")/e2e-test-subrs"
 PATH=$PATH:.:$srcdir
 # Top-level wrapper.
 if [ $# -eq 0 ]; then
-    e2e-test $0 baseline
+    e2e-test "$0" baseline
     exit
 fi
 
@@ -30,5 +31,5 @@ case $1 in
     baseline)
 	baseline;;
     *)
-	fail "unknown test argument %s\n" $1;;
+	fail "unknown test argument %s\n" "$1";;
 esac