Teaches the test framework to probe rarely used prerequistes lazily,
and make use of it for detecting SYMLINKS, CASE_INSENSITIVE_FS and
NKD/NKC MacOS x gotcha.
* jc/test-prereq:
t3910: use the UTF8_NFD_TO_NFC test prereq
test-lib: provide UTF8 behaviour as a prerequisite
t0050: use the SYMLINKS test prereq
t0050: use the CASE_INSENSITIVE_FS test prereq
test-lib: provide case insensitivity as a prerequisite
test: allow prerequisite to be evaluated lazily
test: rename $satisfied to $satisfied_prereq