[ test ] Remove unnecessary messages from stderr

This prevents "rm: cannot remove ...: No such file or directory" errors
by only running rm when the file is actually present. Another potential
solution would be to keep using rm every time, but reroute 2>/dev/null
so that the failure message doesn't get displayed to the user.

tests/node/node004/run

@@ -1,5 +1,5 @@
 . ../../testutils.sh
 
-rm test.buf
+[ -e test.buf ] && rm test.buf
 
 run --cg node Buffer.idr

tests/node/node017/run

@@ -1,6 +1,6 @@
 . ../../testutils.sh
 
-rm -rf testdir
+[ -e testdir ] && rm -rf testdir
 
 run --cg node dir.idr | filter_test_dir
 cat testdir/test.txt

tests/node/node018/run

@@ -1,5 +1,5 @@
 . ../../testutils.sh
 
-rm testout.txt
+[ -e testout.txt ] && rm testout.txt
 
 run --cg node File.idr