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.
https://github.com/ksh93/ksh/runs/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

src/cmd/ksh93/tests/bracket.sh

@@ -265,11 +265,9 @@ test '(' = ')' && err_exit '"test ( = )" should not be true'
 [[ $($SHELL -c "case  Q in ~(Fi)q |  \$'\E') print ok;;esac" 2> /dev/null) == ok ]] || err_exit '~(Fi)q | \E  not working in case command'
 
 locales=$'\n'$(command -p locale -a 2>/dev/null)$'\n'
-for l in C en_US.ISO8859-1 en_US.ISO8859-15 en_US.UTF-8
-do	if	[[ ! $locales == *$'\n'"${l}"$'\n'* ]] ||
-		[[ $("$SHELL" -c "LC_COLLATE=${l}" 2>&1) ]]
-	then	print -u2 "\t${Command}[$LINENO]: warning: cannot test unavailable locale: ${l}"
-		continue
+for l in C C.UTF-8 en_US.ISO8859-1 en_US.ISO8859-15 en_US.UTF-8
+do	if	[[ $l != C* && ( $locales != *$'\n'"${l}"$'\n'* || -n $("$SHELL" -c "LC_COLLATE=${l}" 2>&1) ) ]]
+	then	continue
 	fi
 	export LC_COLLATE=$l
 	set -- \

src/cmd/ksh93/tests/pty.sh

@@ -46,11 +46,14 @@ integer Errors=0 lineno=1
 [[ -d $tmp && -w $tmp && $tmp == "$PWD" ]] || { err\_exit "$LINENO" '$tmp not set; run this from shtests. Aborting.'; exit 1; }
 whence -q pty || { err\_exit "$LINENO" "pty command not found -- tests skipped"; exit 0; }
 
-# On FreeBSD, the stty command does not appear to work correctly on a pty pseudoterminal.
+# On FreeBSD, the OS-provided stty command does not appear to work correctly on a pty pseudoterminal.
 # To avoid a couple of false regressions, we have to set 'erase' and 'kill' on the real terminal.
-stty_restore=$(stty -g) || { err\_exit "$LINENO" "could not save terminal state -- tests skipped"; exit 0; }
-trap 'stty "$stty_restore"' EXIT  # note: on ksh, the EXIT trap is also triggered for termination due to a signal
-stty erase ^H kill ^X
+if	builtin stty 2>/dev/null
+then	PATH=/opt/ast/bin:$PATH
+elif	test -t 1 2>/dev/null 1>/dev/tty && stty_restore=$(stty -g)
+then	trap 'stty "$stty_restore"' EXIT  # note: on ksh, the EXIT trap is also triggered for termination due to a signal
+	stty erase ^H kill ^X
+fi
 
 bintrue=$(whence -p true)