Tests for IO.read().

test/io/read.wren

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+var a = IO.read("a:") // stdin: first
+var b = IO.read("b:") // stdin: second
+IO.print
+// Using write() here because the read lines include the trailing newline.
+IO.write(a)
+IO.write(b)
+
+// Since stdin isn't echoed back to stdout, we don't see the input lines here,
+// and there is no newline between the two prompts since that normally comes
+// from the input itself.
+// expect: a:b:
+// expect: first
+// expect: second

test/io/read_arg_not_string.wren

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+IO.read(null) // expect runtime error: Prompt must be a string.