t/20-config-includes: detect failure to mock fopen
t/20-config-includes: detect failure to mock fopen

Certain setups may not use the lazy resolution necessary for the mocked
fopen to work.  Detect this condition and skip the test gracefully
instead of bailing out.

Signed-off-by: Andrew Gregory <13115e38efea9b5bf08524f6765f5bb44291d47f@gmail.com>

andrewgregory/pacutils,andrewgregory/pacutils