Backend tests: AND, OR, NOT

Here are some really easy codegen tests for logic gates in the VHDL backend.
A lot of these test cases and existing test cases could be written more concisely in the form
```java
int countMatches(List<String> withinArchitecture, String pattern) {...}
...
assertEquals("expect exactly one OR gate in generated code", 
   1, countMatches(withinArchitecture, "(?i)<=.*\\bOR\\b"))
```