[Tests] add try/try_err helpers; convert tests to use them

Add `try` and `try_err` helper functions to `test/common.sh` that capture stdout/stderr and exit code from a single invocation, eliminating duplicate command executions in tests.
Convert all existing tests that used the `OUTPUT`/`EXIT_CODE` double-invocation pattern to use the new helpers.
Also fixes a pre-existing bug in the `nvm_die_on_prefix` test where ASCII apostrophes were used instead of U+2019 to match nvm.sh output.

test/common.sh

@@ -1,3 +1,29 @@
+# Runs a command once and captures stdout and exit code.
+# Suppresses xtrace in the subshell. Discards stderr.
+#
+# Sets: CAPTURED_STDOUT, CAPTURED_EXIT_CODE
+#
+# Usage:
+#   try nvm_version current
+#   [ "$CAPTURED_STDOUT" = "v20.0.0" ] || die "wrong output"
+#   [ "$CAPTURED_EXIT_CODE" = 0 ] || die "wrong exit code"
+try() {
+  CAPTURED_STDOUT="$(set +x; "$@" 2>/dev/null)" && CAPTURED_EXIT_CODE=0 || CAPTURED_EXIT_CODE=$?
+}
+
+# Runs a command once and captures stderr and exit code.
+# Suppresses xtrace in the subshell. Discards stdout.
+#
+# Sets: CAPTURED_STDERR, CAPTURED_EXIT_CODE
+#
+# Usage:
+#   try_err nvm_alias
+#   [ "$CAPTURED_STDERR" = "An alias is required." ] || die "wrong error"
+#   [ "$CAPTURED_EXIT_CODE" = 1 ] || die "wrong exit code"
+try_err() {
+  CAPTURED_STDERR="$(set +x; "$@" 2>&1 >/dev/null)" && CAPTURED_EXIT_CODE=0 || CAPTURED_EXIT_CODE=$?
+}
+
 assert_ok() {
   local FUNCTION=$1
   shift