github: Re-disable Mac CI runner (re: 5c389035)

The Mac runner is still broken: intermittent pipe- and
signal-related regressions that do not occur on any real Mac.
1892358749

.github/workflows/ci.yml:
- Remove the macOS runner.

src/cmd/ksh93/tests/pty.sh:
- Do not skip pty tests if there is no tty. (On FreeBSD with no
  tty, the tty builtin would need to be enabled in builtins.c.)

src/cmd/ksh93/tests/bracket.sh:
- Don't be noisy when skipping unavailable locales.

.github/workflows/ci.yml

@@ -30,30 +30,3 @@ jobs:
         bin/package make &&
         : default regression tests with SHOPTs disabled &&
         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: |
-        PS4="$PS4[ci.yml] "
-        set -o xtrace
-        export TZ=UTC
-        ulimit -n 1024
-        : default regression tests &&
-        bin/shtests &&
-        : regression tests with OS-provided UTF-8 locales &&
-        LANG=nl_NL.UTF-8 bin/shtests --locale --nocompile &&
-        LANG=ja_JP.UTF-8 bin/shtests --locale --nocompile &&
-        : disable most SHOPTs, rebuild ksh &&
-        sed -E -i.orig \
-          '/^SHOPT (2DMATCH|AUDIT|BGX|BRACEPAT|DYNAMIC|EDPREDICT|ESH|FIXEDARRAY|HISTEXPAND|MULTIBYTE|NAMESPACE|OPTIMIZE|SUID_EXEC|STATS|VSH)=/ s/=1/=0/' \
-          src/cmd/ksh93/SHOPT.sh &&
-        bin/package make &&
-        : default regression tests with SHOPTs disabled &&
-        bin/shtests