tests: fix compiler warnings

tests: fix compiler warnings

Detected by icc.  The first three files are real test bugs, the
last merely silences a warning about mixing an int with an enum type.

* tests/test-getopt.h (test_getopt): Fix condition.
* tests/test-getopt_long.h (test_getopt_long): Likewise.
* tests/test-pipe2.c (main): Likewise.
* tests/test-quotearg-simple.c (main): Avoid icc warning.

Signed-off-by: Eric Blake <eblake@redhat.com>