tests: Unify some tmpdir code, add ability for C to use libtest.sh

I want to be able to easily test the C API on actual data in an OSTree
repo.  The shell `libtest.sh` has code to generate it.  Bridge the two
worlds by introducing a little `libostreetest` library which has a C
API which spawns a shell that runs things in `libtest.sh`.

Yes, this is about as beautiful as it sounds, which is to say, it's
not.  But it works!

Note while we were here, I realized we were actually now creating
*two* tmpdirs per test in `make check` because the tap driver was
already doing that.  Unify it so we know the C code can rely on it.

buildutil/tap-test

@@ -1,12 +1,22 @@
 #! /bin/bash
-
-# run a GTest in tap mode. The test binary is passed as $1
+#
+# Run a test in tap mode, ensuring we have a temporary directory.  We
+# always use /var/tmp becuase we might want to use user xattrs, which
+# aren't available on tmpfs.
+#
+# The test binary is passed as $1
 
 srcd=$(cd $(dirname $1) && pwd)
 bn=$(basename $1)
-tempdir=$(mktemp -d)
+tempdir=$(mktemp -d /var/tmp/tap-test.XXXXXX)
+touch ${tempdir}/.testtmp
 function cleanup () {
-    rm "${tempdir}" -rf
+    if test -n "${TEST_SKIP_CLEANUP:-}"; then
+	echo "Skipping cleanup of ${test_tmpdir}"
+    else if test -f ${tempdir}/.test; then
+	rm "${tempdir}" -rf
+    fi
+    fi
 }
 trap cleanup EXIT
 cd ${tempdir}