Earlier, this code was conceived of to test a "function" specifically,
but some future snippet could test multiple function calls, or a
subset of a function call (e.g. snippets/syscall_clock_gettime.c
already only tests one particular syscall diversion number).
Normalizing on the name "snippet" should make it easier to understand
the code going forward.
Most of these snippets are likely to have some env var that causes the
data to hold constant, while the data will otherwise be likely to vary
over time.
This framework offers a way to test those snippets, by dropping the
variable and an example value in the test/snippets/FOO.variable
one-line file.
Note that the test/snippets/syscall.c snippet is *not* expected to
vary over time (or to differ when any given variable is set), so we
simply don't add any test/snippets/syscall.variable file to avoid it
being tested in this way.
