pyquote: Add tests

I initially added pyquote only as debugging tool (in b429839d "tests:
Show pickles in a way that can be copy-pasted into Python"), and later
it started to be used in the Encoder (see 18004fbd "Move pyquote into
main codebase"). However there is no explicit tests for pyquote.

Add some pyquote tests.

pyquote_test.go

@@ -26,6 +26,16 @@ func testCodec(t *testing.T, transform func(in string)(string, error), testv []C
 	}
 }
 
+func TestPyQuote(t *testing.T) {
+	testCodec(t, func(in string) (string, error) {
+		return pyquote(in), nil
+	}, []CodecTestCase{
+		{`\"'`, `"\\\"'"`},
+		{"\x80hello мир", `"\x80hello мир"`},
+		{"\n\r\x01", `"\n\r\x01"`},
+	})
+}
+
 func TestPyDecodeStringEscape(t *testing.T) {
 	testCodec(t, pydecodeStringEscape, []CodecTestCase{
 		{`hello`, "hello"},