Merge branch 'jc/test-prereq'

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