These tests spawn a sub-process with -d extension=phar.so, which emits a warning on stdout when the extension is compiled statically in the php binary.

In glibc, stdout is buffered, and we also don't flush it explicitly. The process is later killed with SIGTERM, so the buffer is never flushed, and the warning is not visible.

With musl, stdout is unbuffered (or maybe line-buffered, or the buffer is smaller), so the warning is visible, causing the tests to fail.

In this PR I add -d extension=phar.so only if phar was compiled as a dynamic module, so that no warning is emitted.

This is out of scope of this PR, but we should probably flush after emitting a warning. Also, I think that warnings go to stdout as part of the display_errors functionality, but ideally they should be written to stderr when using the CLI SAPI.