Assert the parser is at augmented assign token (#10269)

## Summary

This PR updates fixes one of the `FIXME` comment to assert that the
parser is at one of the possible augmented assignment token when parsing
an augmented assignment statement.

## Test Plan

1. Add valid test cases for all the possible augmented assignment tokens
2. Add invalid test cases similar to assignment statement

crates/ruff_python_parser/resources/valid/statement/augmented_assignment.py

@@ -1,3 +1,18 @@
 x += 1
 x.y += (1, 2, 3)
 x[y] += (1, 2, 3)
+
+# All possible augmented assignment tokens
+x += 1
+x -= 1
+x *= 1
+x /= 1
+x //= 1
+x %= 1
+x **= 1
+x &= 1
+x |= 1
+x ^= 1
+x <<= 1
+x >>= 1
+x @= 1