Fix most shell hygiene issues found by shellcheck

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

src/tests/unicode-later-combining.test

@@ -10,11 +10,12 @@
 # that are visually identical.  The variant test is not run or validated.
 #
 
-. $(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 post
+    e2e-test "$0" baseline post
     exit
 fi
 
@@ -36,7 +37,7 @@ variant()
 post()
 {
     export LANG=C
-    if grep -q "$(printf '^\302\240\314\202$')" $(basename $0).d/baseline.capture; then
+    if grep -q "$(printf '^\302\240\314\202$')" "$(basename "$0").d/baseline.capture"; then
 	exit 0
     fi
     exit 1
@@ -50,5 +51,5 @@ case $1 in
     post)
 	post;;
     *)
-	fail "unknown test argument %s\n" $1;;
+	fail "unknown test argument %s\n" "$1";;
 esac