TestUrlRequestCallback: assert internal states using matchers

Currently, TestUrlRequestCallback asserts state transitions through
boolean expressions. When these fail, they simply yell "true != false",
without surfacing the wrong underlying state transition (e.g.,
`next_state == ON_REDIRECT`).

Using assertThat & matchers avoids this problem.

Test: m & test run
Change-Id: Ia26dc0e6f2fe9b53a1f92638ab0b74e39b936d3d