Add tests for verify_overlaps script

Refactor verify_overlaps to make it testable and add tests for the
comparison. It does make one significant change in behavior which is to
read each of the files produced by a bootclasspath_fragment into a dict
before comparison, rather than reading and comparing them a row at a
time. That allows it to reuse the code to read a CSV into a dict.

Bug: 194063708
Test: atest --host verify_overlaps_test
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - manually change files to cause difference in flags to check
        that it detects the differences.
Change-Id: Ib70ac87fe089fc25e3bef18f367d4939bfc0cb8d

scripts/hiddenapi/Android.bp

@@ -83,3 +83,24 @@ python_binary_host {
         },
     },
 }
+
+python_test_host {
+    name: "verify_overlaps_test",
+    main: "verify_overlaps_test.py",
+    srcs: [
+        "verify_overlaps.py",
+        "verify_overlaps_test.py",
+    ],
+    version: {
+        py2: {
+            enabled: false,
+        },
+        py3: {
+            enabled: true,
+            embedded_launcher: true,
+        },
+    },
+    test_options: {
+        unit_test: true,
+    },
+}