[clangd] Switch from EXPECT_TRUE to ASSERT_TRUE in remote marshalling tests
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.
Reviewed By: kadircet
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D84535
Signed-off-by: Kirill Bobyrev <email@example.com>