testsuite: Return error if tty is TTY

In some build systems, such as the Debian pbuilders, the
environment is strange. The tty is called "TTY" which causes
some of the ps tests to fail.

This commit checks for that specific result and returns ""
so the tests can be bypassed.

Replaces Debian patch fix_checks.

References:
 https://salsa.debian.org/debian/procps/-/blob/debian/2%253.3.17-7/debian/patches/fix_checks

testsuite/config/unix.exp

@@ -205,5 +205,10 @@ proc get_tty {} {
         return ""
     }
     regexp "/dev/(.+)" $raw_tty > tty
+    if { $tty == "tty" } {
+        warning "TTY is tty"
+        return ""
+    }
+
     return $tty
 }