Fix IFS construction in test helper scripts

Since command substitution strips away all trailing whitespace, we need
to print a non-whitespace character at the end of the sequence and later
remove it.

test/Xnest.sh

@@ -2,7 +2,8 @@
 
 # Not quite the "unofficial BASH strict mode", but better than nothing.
 set -eu
-IFS="$(printf '\n\t')"
+IFS="$(printf '\t\n.')"
+IFS="${IFS%?}"
 
 # Simple test script to launch the locally-built tint3 and the Openbox window
 # manager in a nested X session.

test/xvfb-run.sh

@@ -2,7 +2,8 @@
 
 # Not quite the "unofficial BASH strict mode", but better than nothing.
 set -eu
-IFS="$(printf '\n\t')"
+IFS="$(printf '\t\n.')"
+IFS="${IFS%?}"
 
 # Simple wrapper around Xvfb, used to run some test programs in a virtual X
 # server. Provides a minimal replacement for xvfb-run which isn't universally