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/functions.sh

@@ -41,6 +41,7 @@ tmp=$(
 	exit 1
 }
 trap 'cd / && rm -rf "$tmp"' EXIT
+binecho=$(whence -p echo)
 
 integer foo=33
 bar=bye
@@ -141,7 +142,7 @@ fi
 if	[[ $PWD != "$dir" ]]
 then	err_exit 'cd inside nested subshell changes $PWD'
 fi
-fun() /bin/echo hello
+fun() "$binecho" hello
 if	[[ $(fun) != hello ]]
 then	err_exit one line functions not working
 fi