The functional2 test suite was broken in the development shell for two mostly
separate reasons leading to "no such file or directory" errors:
* The `BUILD_TEST_SHELL` (already containing the correct path) would always be
set from the Meson option `build_test_shell`, even to the empty string if
that option is not set. Skip the overwrite in this case to make the
environment variable work again.
* Sandboxed builds would fail to find their builder `/bin/sh`, since in the
development shell busybox is not found, so no sandbox shell gets configured.
Use the shell from `BUILD_TEST_SHELL` instead. (How this does not break the
old functional tests remains a mystery.)
Change-Id: I6a6a696424e8caaef3f9b68e3738bfd58ea0b056
Alois Wohlschlager
42691f0d94