Fix --enable-installed-tests when built from a tarball

The Debian package for gjs builds from the official tarball release, and uses --enable-installed-tests so that we can check whether new versions of dependencies like GLib and mozjs cause regressions in gjs (without recompiling gjs, which could hide ABI breakage in dependencies). When I tried upgrading to gjs 1.53.90 with mozjs 60, this build failed:

make[2]: *** No rule to make target 'installed-tests/debugger/backtrace.test', needed by 'all-am'.  Stop.

This can be reproduced with make distcheck DISTCHECK_CONFIGURE_FLAGS=--enable-installed-tests.

In Debian I worked around this by retrieving that file from the 1.53.90 git tag and patching it in, but a nicer solution is to make the tarball more complete (or use Meson, but that isn't a change I can make right now).

With that fixed, make distcheck DISTCHECK_CONFIGURE_FLAGS=--enable-installed-tests still failed because the debugger tests weren't cleaned up, so I've fixed that in the same MR.

It might also be good to set

AM_DISTCHECK_CONFIGURE_FLAGS = --enable-installed-tests

in Makefile.am so that the CI exercises this automatically in future, but I haven't done that in this MR, since it isn't necessary to fix the bug.