Start adding tests for static replacement

common/djangoapps/static_replace/test/test_static_replace.py

+from nose.tools import assert_equals
+from static_replace import replace_static_urls, replace_course_urls
+
+DATA_DIRECTORY = 'data_dir'
+COURSE_ID = 'org/course/run'
+
+
+def test_multi_replace():
+    static_source = '"/static/file.png"'
+    course_source = '"/course/file.png"'
+
+    assert_equals(
+        replace_static_urls(static_source, DATA_DIRECTORY),
+        replace_static_urls(replace_static_urls(static_source, DATA_DIRECTORY), DATA_DIRECTORY)
+    )
+    assert_equals(
+        replace_course_urls(course_source, COURSE_ID),
+        replace_course_urls(replace_course_urls(course_source, COURSE_ID), COURSE_ID)
+    )
+    assert False