Fix test failures in CI build, disable Mac build (#39)

.github/workflows/ci.yml:
- Disable Mac build as the GitHub runners appear to be broken
  (e.g. SIGCHLD fails, unlike on real Macs) and tend to hang.
- For the Linux build:
  - Set GMT timezone for 'printf %T' tests in builtins.sh.
  - Set the ulimit for open files to 1024 as the subshell.sh tests
    need a lot of open files.
  - As the runners lack the POSIX standard /dev/tty device, use the
    script command to provide a fake /dev/tty for the bracket.sh
    tests that use 'test -t $fd'.
    Ref.: https://github.com/actions/runner/issues/241

.github/workflows/ci.yml

@@ -13,15 +13,7 @@ jobs:
     - name: Build
       run: bin/package make
     - name: Regression tests
-      run: bin/shtests
-
-  MacOS:
-    name: MacOS
-    runs-on: macos-latest
-    steps:
-    - name: Checkout sources
-      uses: actions/checkout@v2
-    - name: Build
-      run: bin/package make
-    - name: Regression tests
-      run: bin/shtests
+      run: |
+        export TZ=GMT
+        ulimit -n 1024
+        script -q -e -c "bin/shtests"