Add new driver test for use of outdated .o files

This is something that's quite important for the correctness of the
incremental build system and doesn't appear to be tested currently; this
test fails on my hashing branch, whereas all of the other (non-perf)
tests pass.

testsuite/tests/driver/recomp019/B1.hs

+module B where
+
+x :: Int
+x = 2

testsuite/tests/driver/recomp019/B2.hs

+module B where
+
+x :: Int
+x = 12

testsuite/tests/driver/recomp019/C.hs

+module C where
+
+x :: Int
+x = 3

testsuite/tests/driver/recomp019/Main.hs

+module Main where
+
+import qualified B
+import qualified C
+
+main = putStrLn $ show $ B.x + C.x

testsuite/tests/driver/recomp019/Makefile

+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+# Recompilation test for when .hi files are up to date but .o files are
+# not
+
+clean:
+	rm -f Main *.o *.hi B.hs
+
+recomp019: clean
+	cp B1.hs B.hs
+	echo 'first run'
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make Main
+	./Main
+	
+	# Update B.hs
+	cp B2.hs B.hs
+	
+	# Update B.hi (but not B.o)
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make B -fno-code -fwrite-interface
+	
+	# Recompile
+	echo 'second run'
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make Main
+	./Main

testsuite/tests/driver/recomp019/all.T

+# Recompilation test for when .hi files are up to date but .o files are
+# not
+
+test('recomp019',
+     [extra_files(['Main.hs', 'B1.hs', 'B2.hs', 'C.hs'])],
+     makefile_test, [])

testsuite/tests/driver/recomp019/recomp019.stdout

+first run
+[1 of 3] Compiling B                ( B.hs, B.o )
+[2 of 3] Compiling C                ( C.hs, C.o )
+[3 of 3] Compiling Main             ( Main.hs, Main.o )
+Linking Main ...
+5
+[1 of 1] Compiling B                ( B.hs, nothing )
+second run
+[2 of 3] Compiling B                ( B.hs, B.o )
+Linking Main ...
+15