regress tests: remove use of unportable direct paths to commands

Many tests used direct paths to some commands, mostly /bin/echo and
/bin/cat. This is unportable (breaks on e.g. NixOS).
The correct way is to obtain the direct path using 'whence -p'.

There was also one use of '/usr/bin/pstack' in tests/comvario.sh
that seemed bogus. Apparently this was supposed to analyse a core
file after a crash. Even on Solaris and Linux, where that command
exists, the argument is documented to be a PID, not a core file. If
this ever worked anywhere, then it was system-specific enough to be
useless here, so I've removed it.

(cherry picked from commit 4563b8bc651cd9cb18dc73f56a041f7ac5534395)

src/cmd/ksh93/tests/signal.sh

@@ -283,18 +283,18 @@ then	float s=$SECONDS
 	(( SECONDS-s < 4 )) && err_exit 'parent completes early'
 fi
 
-yes=$(whence -p yes)
-if	[[ $yes ]]
-then	for exp in TERM VTALRM PIPE
+yes() for ((;;)); do print y; done
+
+	for exp in TERM VTALRM PIPE
 	do	if	[[ ${SIG[$exp]} ]]
 		then	{
-				$SHELL <<- EOF
+				bindate=$(whence -p date) "$SHELL" <<- EOF
 				foo() { return 0; }
 				trap foo EXIT
 				{ sleep 2; kill -$exp \$\$; sleep 3; kill -0 \$\$ && kill -KILL \$\$; } &
-				$yes |
+				yes |
 				while read yes
-				do	(/bin/date; sleep .1)
+				do	("\$bindate"; sleep .1)
 				done > /dev/null
 				EOF
     			} 2>> /dev/null
@@ -302,7 +302,6 @@ then	for exp in TERM VTALRM PIPE
     			[[ $exp == $got ]] || err_exit "kill -$exp \$\$ failed, required termination by signal '$got'"
 		fi
 	done
-fi
 
 SECONDS=0
 $SHELL 2> /dev/null -c 'sleep 2 && kill $$ & trap "print done; exit 3" EXIT; (sleep 5); print finished' > $tmp/sig