[clangd] Switch from EXPECT_TRUE to ASSERT_TRUE in remote marshalling tests

Summary:
When dereferencing Optional's it makes sense to use ASSERT_TRUE for better
test failures readability. Switch from EXPECT_TRUE to ASSERT_TRUE where
it is appropriate.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84535



Signed-off-by: Kirill Bobyrev <kbobyrev@google.com>