* document ProgramTest::new
* simplify ProgramTest::new doc-string
* make ProgramTest::add_program noisier
`add_program` (and `new`, implicitly) now prints a warning when the user
supplies a bogus program name to a ProgramTest and invokes `test-bpf`.
Additionally, it is now impossible to ask for a regular `test` and for
the generated ProgramTest to load BPF code instead of native code.
Previously, this was caused by a precedence issue: BPF code would always
be preferred over native if the program name was valid, regardless of
user choice.
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