test: add a case of 'general newilne' AKA \R syntax

Checking support for multiple \R, the "general newline" shorthand
https://github.com/kkos/oniguruma/blob/3bbb3d65c65350bfd941129592cb390aadedb25a/doc/SYNTAX.md#23-onig_syn_op2_esc_capital_r_general_newline-enable-r

regex_test.go

@@ -606,6 +606,15 @@ var findTests = []FindTest{
 			20, 21, 21, 22, 22, 23, 23, 24, 24, 25, 25, 26, 26, 27, 27, 28, 28, 29, 29, 30,
 			30, 31, 31, 32, 32, 33, 33, 34, 34, 35, 35, 36),
 	},
+
+	// https://github.com/kkos/oniguruma/blob/2a25d8c2c4699c57f4d456ccd13149c0c714723d/test/test_back.c#LL629C1-L631C25
+	{"\\R", "\r\n", build(1, 0, 2)},
+	{"\\R", "\r", build(1, 0, 1)},
+	{"\\R", "\n", build(1, 0, 1)},
+
+	{`b\R.*\R`, "ab\nc\n", build(1, 1, 5)},
+	// equivalent, from Linguist dataset
+	{`^\s*BEGIN\R\s*.*\R`, "\tBEGIN\n\tMultiUse = -1  'True\n", build(1, 0, 29)},
 }
 
 // build is a helper to construct a [][]int by extracting n sequences from x.