Fix test-no_std.sh to handle the case where topdir is ".".

Instead of using "cd $topdir" to get back to the top, use popd.

cranelift/test-no_std.sh

@@ -14,10 +14,9 @@ function banner {
 
 # Test those packages which have no_std support.
 LIBS="codegen frontend wasm native module simplejit umbrella"
-cd "$topdir"
 for LIB in $LIBS; do
     banner "Rust unit tests in $LIB"
-    cd "lib/$LIB"
+    pushd "lib/$LIB" >/dev/null
 
     # Test with just "core" enabled.
     cargo +nightly test --no-default-features --features core
@@ -25,7 +24,7 @@ for LIB in $LIBS; do
     # Test with "core" and "std" enabled at the same time.
     cargo +nightly test --features core
 
-    cd "$topdir"
+    popd >/dev/null
 done
 
 banner "OK"