Minor: Assert test_enabled_backtrace requirements to run

[comphead]

Which issue does this PR close?

Closes #.

Rationale for this change

If the test_enabled_backtrace launched without RUST_BACKTRACE=1 the test will fail with the reason which require some time to investigate.

What changes are included in this PR?

Add proper error message if assertion doesn't satisfy prior to test run

Are these changes tested?

Are there any user-facing changes?

[alamb]

I verified that when i run

cargo test -p datafusion-common --all-features

With this branch it fails with a useful message:

---- error::test::test_enabled_backtrace stdout ----
thread 'error::test::test_enabled_backtrace' panicked at datafusion/common/src/error.rs:671:18:
Environment variable RUST_BACKTRACE must be set to 1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

And it runs well with that variable set:

andrewlamb@Andrews-MBP-2:~/Software/datafusion$ RUST_BACKTRACE=1 cargo test -p datafusion-common --all-features -- backtrace
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.10s
     Running unittests src/lib.rs (target/debug/deps/datafusion_common-87ffe852e4faebf2)

running 1 test
test error::test::test_enabled_backtrace ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 205 filtered out; finished in 0.03s

   Doc-tests datafusion_common

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 42 filtered out; finished in 0.00s

On main the error is not helpful:

andrewlamb@Andrews-MBP-2:~/Software/datafusion$ cargo test -p datafusion-common --all-features -- backtrace
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.09s
     Running unittests src/lib.rs (target/debug/deps/datafusion_common-87ffe852e4faebf2)

running 1 test
test error::test::test_enabled_backtrace ... FAILED

failures:

---- error::test::test_enabled_backtrace stdout ----
thread 'error::test::test_enabled_backtrace' panicked at datafusion/common/src/error.rs:671:9:
assertion failed: err.contains(DataFusionError::BACK_TRACE_SEP)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    error::test::test_enabled_backtrace

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 209 filtered out; finished in 0.00s

error: test failed, to rerun pass `-p datafusion-common --lib`

fyi @findepi

[alamb]

👍

[findepi]

thank you!