testing: fix strace color probing in allow-debuggers & seccomp-ptrace

if strace runs in a terminal, it probes the background color to select
its color palette. this probing expects a reply, but due to expect
intercepting the io, the answer isn't sent back to strace, so it never
starts printing the expected output.

test/environment/allow-debuggers.exp

@@ -5,6 +5,10 @@
 
 set timeout 10
 cd /home
+
+# disable strace's terminal background color probing
+set env(NO_COLOR) 1
+
 spawn $env(SHELL)
 match_max 100000
 

test/filters/seccomp-ptrace.exp

@@ -4,6 +4,10 @@
 # License GPL v2
 
 set timeout 10
+
+# disable strace's terminal background color probing
+set env(NO_COLOR) 1
+
 spawn $env(SHELL)
 match_max 100000