Extract line reading logic to reader and remove REPL tests

Reading a line into the buffer was the only logic complex enough to be
worth testing really, and exposing the necessary parts of it to test
it effectively was a pain.  Makes more sense to move read_line out and
throw away most of the tests.

tests/reader_tests.c

@@ -1,8 +1,21 @@
 #include "reader.h"
 #include "testing.h"
 
+#include <stdio.h>
+
 static memory_pool_t pool;
 
+static char *test_input;
+static int test_input_len;
+
+static int test_get_byte(void)
+{
+	if (test_input_len < 0)
+		return EOF;
+	--test_input_len;
+	return *test_input++;
+}
+
 static void input_1234_is_read_as_number_with_expected_value(void)
 {
 	init_memory_pool(&pool);
@@ -234,6 +247,41 @@ static void trailing_spaces_are_ignored(void)
 	ASSERT_EQUAL(14, result->number);
 }
 
+static void read_line_copies_line_into_buffer(void)
+{
+	test_input = "foobar\nbazquux";
+	test_input_len = strlen(test_input);
+	char buffer[10];
+	read_line(test_get_byte, buffer, sizeof(buffer));
+	ASSERT_MEM_EQUAL("foobar", buffer, 6);
+}
+
+static void read_line_returns_length_of_line(void)
+{
+	test_input = "foobar\nbazquux";
+	test_input_len = strlen(test_input);
+	char buffer[10];
+	const int len = read_line(test_get_byte, buffer, sizeof(buffer));
+	ASSERT_EQUAL(6, len);
+}
+
+static void read_line_returns_minus_1_on_eof(void)
+{
+	test_input_len = 0;
+	char buffer[10];
+	const int len = read_line(test_get_byte, buffer, sizeof(buffer));
+	ASSERT_EQUAL(-1, len);
+}
+
+static void read_line_returns_buffer_size_if_no_newline(void)
+{
+	test_input = "foobarbazquux";
+	test_input_len = strlen(test_input);
+	char buffer[10];
+	const int len = read_line(test_get_byte, buffer, sizeof(buffer));
+	ASSERT_EQUAL(10, len);
+}
+
 int main(void)
 {
 	TESTING_BEGIN();
@@ -256,5 +304,9 @@ int main(void)
 	RUN_TEST(spaces_inside_parens_are_ignored);
 	RUN_TEST(leading_spaces_are_ignored);
 	RUN_TEST(trailing_spaces_are_ignored);
+	RUN_TEST(read_line_copies_line_into_buffer);
+	RUN_TEST(read_line_returns_length_of_line);
+	RUN_TEST(read_line_returns_minus_1_on_eof);
+	RUN_TEST(read_line_returns_buffer_size_if_no_newline);
 	TESTING_END();
 }