test: make Result-returning tests produce a line number

Without this change
when the test returns a `Result`, `cargo test` does not show
the line number.

To make the tests as easy to debug as the panicking tests,
enable `backtrace` feature on `anyhow` and add debug information
to add source line numbers to backtraces.

Now running `RUST_BACKTRACE=1 cargo test` produces backtraces
with the line numbers. For example:

Error: near ",": syntax error in SELECT COUNT(,,*) FROM msgs_status_updates; at offset 13

Caused by:
    Error code 1: SQL error or missing database

Stack backtrace:
   0: <core::result::Result<T,F> as core::ops::try_trait::FromResidual<core::result::Result<core::convert::Infallible,E>>>::from_residual
             at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/result.rs:1962:27
   1: deltachat::sql::Sql::query_row::{{closure}}::{{closure}}
             at ./src/sql.rs:466:23
   2: deltachat::sql::Sql::call::{{closure}}::{{closure}}
             at ./src/sql.rs:379:55
   3: tokio::runtime::context::runtime_mt::exit_runtime
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/context/runtime_mt.rs:35:5
   4: tokio::runtime::scheduler::multi_thread::worker::block_in_place
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/scheduler/multi_thread/worker.rs:438:9
   5: tokio::runtime::scheduler::block_in_place::block_in_place
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/scheduler/block_in_place.rs:20:5
   6: tokio::task::blocking::block_in_place
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/task/blocking.rs:78:9
   7: deltachat::sql::Sql::call::{{closure}}
             at ./src/sql.rs:379:19
   8: deltachat::sql::Sql::query_row::{{closure}}
             at ./src/sql.rs:469:10
   9: deltachat::sql::Sql::count::{{closure}}
             at ./src/sql.rs:443:76
  10: deltachat::webxdc::tests::change_logging_webxdc::{{closure}}
             at ./src/webxdc.rs:2644:18
  11: <core::pin::Pin<P> as core::future::future::Future>::poll
             at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/future/future.rs:125:9
  12: tokio::runtime::park::CachedParkThread::block_on::{{closure}}
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/park.rs:282:63
  13: tokio::runtime::coop::with_budget
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/coop.rs:107:5
      tokio::runtime::coop::budget
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/coop.rs:73:5
      tokio::runtime::park::CachedParkThread::block_on
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/park.rs:282:31
  14: tokio::runtime::context::blocking::BlockingRegionGuard::block_on
             at /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.34.0/src/runtime/context/blocking.rs:66:9
...

Line 10 of the backtrace contains the line number in the test (2644).

CONTRIBUTING.md

@@ -91,6 +91,12 @@ All errors should be handled in one of these ways:
 - With `.log_err().ok()`.
 - Bubbled up with `?`.
 
+`backtrace` feature is enabled for `anyhow` crate
+and `debug = 1` option is set in the test profile.
+This allows to run `RUST_BACKTRACE=1 cargo test`
+and get a backtrace with line numbers in resultified tests
+which return `anyhow::Result`.
+
 ### Logging
 
 For logging, use `info!`, `warn!` and `error!` macros.

Cargo.toml

@@ -11,6 +11,10 @@ panic = 'abort'
 opt-level = 1
 
 [profile.test]
+# Make anyhow `backtrace` feature useful.
+# With `debug = 0` there are no line numbers in the backtrace
+# produced with RUST_BACKTRACE=1.
+debug = 1
 opt-level = 0
 
 # Always optimize dependencies.
@@ -99,6 +103,7 @@ uuid = { version = "1", features = ["serde", "v4"] }
 
 [dev-dependencies]
 ansi_term = "0.12.0"
+anyhow = { version = "1", features = ["backtrace"] } # Enable `backtrace` feature in tests.
 criterion = { version = "0.5.1", features = ["async_tokio"] }
 futures-lite = "2.0.0"
 log = "0.4"