Fix $abs_top_srcdir in test_rust.sh

Consistently use ../../.. as a fallback for $abs_top_srcdir in
test_rust.sh.

Fixes bug 27093; bugfix on 0.3.4.3-alpha.

changes/bug27093

@@ -0,0 +1,3 @@
+  o Minor bugfixes (rust):
+    - Consistently use ../../.. as a fallback for $abs_top_srcdir in
+      test_rust.sh. Fixes bug 27093; bugfix on 0.3.4.3-alpha.

src/test/test_rust.sh

@@ -3,7 +3,7 @@
 
 set -e
 
-export LSAN_OPTIONS=suppressions=${abs_top_srcdir}/src/test/rust_supp.txt
+export LSAN_OPTIONS=suppressions=${abs_top_srcdir:-../../..}/src/test/rust_supp.txt
 
 for cargo_toml_dir in "${abs_top_srcdir:-../../..}"/src/rust/*; do
     if [ -e "${cargo_toml_dir}/Cargo.toml" ]; then