CI: Also run on macos-latest

Make the `test/run` script compatible to the ancient Bash version on macos.

```
test/run: line 56: conditional binary operator expected
```

Also, run `set -e` only for Bash >= 4. Otherwise the shell silently exits with
a failure when a command (expectedly) fails.

fix bogus test failure with Bash 3.2 on macos

.github/workflows/ruby.yml

@@ -16,10 +16,12 @@ on:
 jobs:
   test:
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+
     strategy:
       matrix:
         ruby-version: ['2.6', '2.7', '3.0']
+        os: [ubuntu-latest, macos-latest]
 
     steps:
     - uses: actions/checkout@v2

test/run

@@ -1,6 +1,10 @@
 #! /usr/bin/env bash
 
-set -eEuCo pipefail
+set -EuCo pipefail
+
+if [[ "${BASH_VERSINFO[0]}" -ge 4 ]]; then
+  set -e
+fi
 
 declare RED=$'\033[31m'
 declare GREEN=$'\033[32m'
@@ -53,7 +57,7 @@ function XFAIL() {
     echo "$RED" "FAIL$RESET - $* (unexpected success, ${BASH_SOURCE[1]}:${BASH_LINENO[0]})" >&2
   else
     ret=$?
-    if [[ -v expected && $expected -ne $ret ]]; then
+    if [[ -n "$expected" && $expected -ne $ret ]]; then
       ((++ERRORS))
 
       echo "$RED" "FAIL$RESET - $* (expected: $expected got $ret, ${BASH_SOURCE[1]}:${BASH_LINENO[0]})" >&2