tests: avoid skipping by fixing build of shared libraries

tests: avoid skipping by fixing build of shared libraries

Two df(1) tests were skipped (since commit ee367bd38dac), because
the build of the shared library in those tests failed.

  + gcc -Wall -shared --std=gnu99 -fPIC -O2 k.c -o k.so -ldl
  k.c: In function 'open':
  k.c:37:7: error: implicit declaration of function 'streq'; did you \
  mean 'strsep'? [-Wimplicit-function-declaration]
     37 |   if (streq (path, "/proc/self/mountinfo"))
        |       ^~~~~
        |       strsep

Gnulib streq is not available in the tests.

* tests/df/no-mtab-status.sh: Replace "streq" by "0==strcmp" in the
shared library source.
* tests/df/skip-duplicates.sh: Likewise.