[gtest] Consume `AssertionResult`s in `//c/c/extensions/permissions/`

Wrap each `testing::AssertionResult` in an `EXPECT_TRUE(...)` [0] to
actually check if the assertion passed. The dropped assertion causes a
build failure [1] with a recent Googletest revision that marks
`AssertionResult` as `[[nodiscard]]` [2].

[0]: https://google.github.io/googletest/advanced.html#predicate-assertions-for-better-error-messages
[1]: https://ci.chromium.org/ui/p/chromium/builders/try/linux-rel/2130794/overview
[2]: https://chromium.googlesource.com/external/github.com/google/googletest.git/+/3fbe4db9a39291ae8d7a9c5f1d75896bb4c5a18f%5E%21/

Bug: 398298306
Change-Id: If6b96b50b426cf7ff76e41d4ca2ac28e53d03f3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6303081
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1425193}