Three tests run whichever `layup` is first on PATH, not the one under test

[matthewholman]

test_predict.py:223, test_predict.py (get_onsky_data) and
test_comet.py:216 invoke the CLI as a subprocess:

result = subprocess.run(["layup", "predict", str(input_file), ...])
assert result.returncode == 0

"layup" resolves against PATH, so the test exercises whatever installation
comes first -- not necessarily the working tree. On this machine that is a conda
env with a broken assist/rebound link, and all three fail with
assert 1 == 0 and an unrelated OSError: dlopen ... librebound...so. Putting
the intended environment's bin first makes all three pass, with no code change.

The failure mode is the bad one: it looks like a code regression, and the real
message is buried in captured stderr. It can also pass for the wrong reason,
testing a stale install while the working tree is broken.

Suggest [sys.executable, "-m", ...], or resolving the entry point relative to
sys.executable, so the subprocess inherits the interpreter under test.