[Signin] Prefer GetRefreshTokenForTest() over GetRefreshToken() in tests

Updates `MutableProfileOAuth2TokenServiceDelegateTest` to use the public
testing API `GetRefreshTokenForTest()` instead of accessing the private
`GetRefreshToken()` method directly

This change also cleans up several assertions by replacing
`EXPECT_STREQ` and `.c_str()` calls with `EXPECT_EQ` on the returned
`std::string`.

Change-Id: Iec88bc1fc15de40ac52c4f8012adcf329f9fb85c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7653497
Commit-Queue: Ali Hijazi <ahijazi@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Ernest Nguyen Hung <ernn@google.com>
Cr-Commit-Position: refs/heads/main@{#1598289}