tests: Fix browser.sh path detection to be less STI specific

Only set `$SOURCE` to ./source/ if that actually exists, which is when
it is being called through Standard Test Interface.

Provide fallbacks for using FMF [1] or when calling the script manually.

[1] https://github.com/cockpit-project/cockpit-podman/pull/649

tests/browser.sh

@@ -1,10 +1,14 @@
 #!/bin/sh
 set -eux
 
-# from standard-test-source
-SOURCE="$(pwd)/source"
+TESTS="$(realpath $(dirname "$0"))"
+if [ -d source ]; then
+    # path for standard-test-source
+    SOURCE="$(pwd)/source"
+else
+    SOURCE="$(realpath $TESTS/..)"
+fi
 LOGS="$(pwd)/logs"
-TESTS="$(pwd)/tests"
 mkdir -p "$LOGS"
 chmod a+w "$LOGS"