testsuite: Use explicit -e in sed command

Otherwise Darwin's sed interprets the command as the suffix for -i.

testsuite/tests/ghci/should_run/T15369.script

@@ -8,6 +8,6 @@
 :all-types
 :l *T15369.hs
 :all-types
-:! sed -i 's/Int/Double/g' T15369.hs
+:! sed -i -e 's/Int/Double/g' T15369.hs
 :l *T15369.hs
 :all-types