tests: make the dependency patching test more complete

- dependency patching test didn't attempt to apply patches; just to see
  whether they were on the spec.

- it applies the patch now and verifies that that patch was applied.

var/spack/repos/builtin.mock/packages/patch-a-dependency/foo.patch

@@ -1 +0,0 @@
-foo

var/spack/repos/builtin.mock/packages/patch-a-dependency/libelf.patch

@@ -0,0 +1,11 @@
+--- patch-a-dependency/configure	2018-08-13 23:13:51.000000000 -0700
++++ patch-a-dependency/configure.patched	2018-08-13 23:14:15.000000000 -0700
+@@ -2,7 +2,7 @@
+ prefix=$(echo $1 | sed 's/--prefix=//')
+ cat > Makefile <<EOF
+ all:
+-	echo Building...
++	echo Patched!
+ 
+ install:
+ 	mkdir -p $prefix

var/spack/repos/builtin.mock/packages/patch-a-dependency/package.py

@@ -33,7 +33,7 @@ class PatchADependency(Package):
 
     version('1.0', '0123456789abcdef0123456789abcdef')
 
-    depends_on('libelf', patches=patch('foo.patch'))
+    depends_on('libelf', patches=patch('libelf.patch'))
 
     def install(self, spec, prefix):
         pass