warn if no test items found

[nickrobinson251]

It's always seemed slightly odd that running 0 tests looks so similar to successfully running some tests, and (i suspect) this could also confuse our AI overlords tools like Claude Code (as suggested at https://youtu.be/nfOVgz_omlU?feature=shared&t=1168).

[nickrobinson251]

Logs a warning:

Before (master)

julia> runtests("test/testfiles/_happy_tests.jl"; name="blahahah_nope")
[ Info: Scanning for test items in project `ReTestItems` at paths: test/testfiles/_happy_tests.jl
[ Info: Finished scanning for test items in 0.04 seconds. Scheduling 0 tests on pid 61468
[ Tests Completed: 0/0 test items were run.
Test Summary: |Time
ReTestItems   | None  0.0s

This PR:

julia> runtests("test/testfiles/_happy_tests.jl"; name="blahahah_nope")
[ Info: Scanning for test items in project `ReTestItems` at paths: test/testfiles/_happy_tests.jl
[ Info: Finished scanning for test items in 0.06 seconds.
┌ Warning: No test items found.
└ @ ReTestItems ~/repos/ReTestItems.jl/src/ReTestItems.jl:381

This is also more consistent with the behaviour we see when the specified file doesn't exist when validate_paths=false (the default):

julia> runtests("test/no_such_tests.jl")
┌ Warning: No such path "test/no_such_tests.jl"
└ @ ReTestItems ~/repos/ReTestItems.jl/src/ReTestItems.jl:121

[nickrobinson251]

follow-up PR to make this throw an error, but that can come next: #216