Enhancements and improved documentation to the byte-code branch coverage

Enhancements and improved documentation to the byte-code branch coverage
testing logic.  Provide new macros that allow the code to specify that
some branch instructions can never take the NULL path and that the OP_Jump
opcode is only interested in equal/not-equal.  The
SQLITE_TESTCTRL_VDBE_COVERAGE file control callback now works slightly
differently (it provides the callback with a bitmask of the branch action,
rather than an integer).

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