testdata: scope GODEBUG to a single test case

I've tripped over this GODEBUG env var four or five times already.
Since it affects any Go program by making them print tons of runtime
information, it also affects "go env", breaking garble horribly.

To prevent further issues, unset the env var when done.

testdata/scripts/tiny.txt

@@ -12,6 +12,9 @@ stderr '^caller: \?\? 1$' # position info is removed
 stderr '^recovered: ya like jazz?'
 ! stderr 'panic: oh noes' # panics are hidden
 
+# Ensure further commands don't have weirdness due to GODEBUG.
+env GODEBUG=''
+
 [short] stop # no need to verify this with -short
 
 # Default mode