test.sh: Set RUST_LIB_BACKTRACE=0 so debug_assertions tests don…

…'t take forever due to capturing a ton of backtraces (#967)

We still want `RUST_BACKTRACE=1` to print a backtrace on panicking, as
that's very useful and doesn't slow tests down. But that also causes
`Backtrace::capture()` to actually capture a backtrace, and since we do
that in every `DisjointMut::{index,index_mut}` call, it makes the tests
extremely slow. Setting `RUST_LIB_BACKTRACE=0` overrides this and makes
the tests fast (enough) again. And it can always be overridden again on
the command line.

.github/workflows/test.sh

@@ -43,7 +43,17 @@ test_args=(
     --suite testdata-multi
 )
 
+# We want `RUST_BACKTRACE=1` in order to print a backtrace on panicking,
+# as that's very useful and doesn't slow tests down.
+# But that also causes `Backtrace::capture()`` to actually capture a backtrace,
+# and since we do that in every `DisjointMut::{index,index_mut}` call,
+# it makes the tests extremely slow.
+# Setting `RUST_LIB_BACKTRACE=0` overrides this and makes the tests fast (enough) again.
+# And it can always be overridden again on the command line.
+#
+# See more in the docs for `Backtrace::capture`.
 export RUST_BACKTRACE=1
+export RUST_LIB_BACKTRACE=${RUST_LIB_BACKTRACE:-0}
 
 if [[ -z $seek_stress_test_rust_path ]]; then
     : # stress test binary not provided; don't include seek stress tests