Add test

Signed-off-by: Nicolás Ojeda Bär <n.oje.bar@gmail.com>

test/blackbox-tests/test-cases/dune-cache/clear.t

@@ -0,0 +1,50 @@
+Test for the "dune cache clear" command.
+
+  $ export DUNE_CACHE=enabled
+  $ export DUNE_CACHE_ROOT=$PWD/dune-cache
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.10)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (rule (with-stdout-to foo (progn)))
+  > EOF
+
+  $ dune build
+
+  $ find $DUNE_CACHE_ROOT -type d | sort -u
+  $TESTCASE_ROOT/dune-cache
+  $TESTCASE_ROOT/dune-cache/files
+  $TESTCASE_ROOT/dune-cache/files/v4
+  $TESTCASE_ROOT/dune-cache/files/v4/11
+  $TESTCASE_ROOT/dune-cache/files/v4/4d
+  $TESTCASE_ROOT/dune-cache/files/v4/b8
+  $TESTCASE_ROOT/dune-cache/meta
+  $TESTCASE_ROOT/dune-cache/meta/v5
+  $TESTCASE_ROOT/dune-cache/meta/v5/2f
+  $TESTCASE_ROOT/dune-cache/meta/v5/39
+  $TESTCASE_ROOT/dune-cache/meta/v5/4d
+  $TESTCASE_ROOT/dune-cache/temp
+  $TESTCASE_ROOT/dune-cache/values
+  $TESTCASE_ROOT/dune-cache/values/v3
+
+  $ dune cache clear
+
+  $ ! test -d $DUNE_CACHE_ROOT
+
+Next let us add some extra directories/files and check that they are not deleted
+by mistake.
+
+  $ dune build
+
+  $ mkdir -p $DUNE_CACHE_ROOT/extra; touch $DUNE_CACHE_ROOT/extra1 $DUNE_CACHE_ROOT/extra/extra2
+
+  $ dune cache clear
+  Error:
+  rmdir($TESTCASE_ROOT/dune-cache): Directory not empty
+  [1]
+
+  $ find $DUNE_CACHE_ROOT -type f | sort -u
+  $TESTCASE_ROOT/dune-cache/extra/extra2
+  $TESTCASE_ROOT/dune-cache/extra1