Read a borrowed const char * return without freeing it

[estebanzimanyi]

The Utf8 string marshaller frees the pointer a char * return hands back.
That is right for a string MEOS mallocs for the caller and fatal for the
const char * it returns out of a static table: temporal_interp answers
with an element of MEOS_INTERPTYPE_NAMES, and freeing that ends the
process — Test host process crashed : free(): invalid pointer. The
eleven functions whose return is const char * — temporal_interp,
temporal_subtype, temporal_basetype_name, meostype_name and their
kin — come back as the pointer itself and are read with
Marshal.PtrToStringUTF8, which copies and frees nothing.

Three tests read an interpolation name: off a linear sequence, off a
discrete one, and off two temporal values in turn. They exercise the native
library, so the test step takes the LD_LIBRARY_PATH of the prefix the
provisioning built, as the smoke step does, and every test class opens a
MEOS session of its own: MEOS keeps the timezone its text I/O goes through
and the error handler per thread, and the test host runs a test on
whichever thread it has free, so a session opened once for the assembly
reaches the thread that opened it and no other.