[XLA:GPU] Tweak cudnn_fused_conv_rewriter_test.

- Move it into the same directory as the implementation (to match idiom of most
  of the rest of the code).
- Use gmock matchers for substr.
- Use constants for cudnn custom calls rather than the raw strings (in case we
  ever have to change them).

No functional change.

PiperOrigin-RevId: 217246977