Functions that may panic can be annotated with `#[track_caller]` so that
in the event of a panic, the function where the user called the
panicking function is shown instead of the file and line within Tokio
source.

This change adds track caller to all the non-unstable public APIs in
Tokio core where the documentation describes how the function may panic
due to incorrect context or inputs.  Since each internal function needs
to be annotated down to the actual panic, it makes sense to start in
Tokio core functionality.

Tests are needed to ensure that all the annotations remain in place in
case internal refactoring occurs.

The test installs a panic hook to extract the file location from the
`PanicInfo` struct and clone it up to the outer scope to check that the
panic was indeed reported from within the test file.  The downside to
this approach is that the panic hook is global while set and so we need
a lot of extra functionality to effectively serialize the tests so that
only a single panic can occur at a time.

The annotation of `block_on` was removed as it did not work. It appears
to be impossible to correctly chain track caller when the call stack to
the panic passes through clojures, as the track caller annotation can
only be applied to functions. Also, the panic message itself is very
descriptive.

Based on feedback (thank-you!), the timer in the into_panic test has
been replaced with `futures::future::pending()` and the lazy_static
dev-dependency has been removed and replaced with `const_mutex` from
`parking_lot`.

Additionally, the panic catching has been reworked a little bit to set
and unset the panic hook around the part of the test where the panic is
expected. This fixes the issue that the normal test output for a failed
test was being eaten up by the custom panic hook.

Caught by the clippy lint.

As suggested in review, instead of wrapping it in
async+await.

Co-authored-by: Alice Ryhl <aliceryhl@google.com>

I noticed that no other tests in tokio are prefixed with
`test_`, so I removed it from the new ones added.