disk: separate most tests from package

Separate most disk tests into internal and external ones.

- disk_test.go: was internal (package disk) and is now external (package
  disk_test)
- Function `entityPath()` and method
  `PartitionTable.findDirectoryEntityPath()` are now exported for tests
  in export_test.go
- Test partition tables moved to partition_table_internal_test.go and
  exported.
- Unique string tests moved to new file: disk_internal_test.go

pkg/disk/disk_internal_test.go

@@ -0,0 +1,108 @@
+package disk
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenUniqueString(t *testing.T) {
+	type testCase struct {
+		base     string
+		existing map[string]bool
+		exp      string
+	}
+
+	testCases := map[string]testCase{
+		"simple": {
+			base: "root",
+			existing: map[string]bool{
+				"one": true,
+				"two": true,
+			},
+			exp: "root",
+		},
+		"collision": {
+			base: "root",
+			existing: map[string]bool{
+				"one":  true,
+				"two":  true,
+				"root": true,
+			},
+			exp: "root00",
+		},
+		"collision-2": {
+			base: "word",
+			existing: map[string]bool{
+				"word":   true,
+				"word00": true,
+				"word01": true,
+				"other":  true,
+			},
+			exp: "word02",
+		},
+	}
+
+	for name := range testCases {
+		tc := testCases[name]
+		t.Run(name, func(t *testing.T) {
+			assert := assert.New(t)
+			out, err := genUniqueString(tc.base, tc.existing)
+			assert.NoError(err)
+			assert.Equal(tc.exp, out)
+		})
+	}
+}
+
+func TestGenUniqueStringManyCollisions(t *testing.T) {
+	type testCase struct {
+		base        string
+		ncollisions int
+		exp         string
+		errmsg      string
+	}
+
+	testCases := map[string]testCase{
+		"baseword33": {
+			base:        "baseword",
+			ncollisions: 33,
+			exp:         "baseword33",
+		},
+		"somany99": {
+			base:        "somany",
+			ncollisions: 99,
+			exp:         "somany99",
+		},
+		"tk42102": {
+			base:        "tk421",
+			ncollisions: 2,
+			exp:         "tk42102",
+		},
+		"so-many-collisions": {
+			base:        "all-the-collisions",
+			ncollisions: 100,
+			errmsg:      `name collision: could not generate unique version of "all-the-collisions"`,
+		},
+	}
+
+	for name := range testCases {
+		tc := testCases[name]
+		t.Run(name, func(t *testing.T) {
+			assert := assert.New(t)
+			existing := map[string]bool{
+				tc.base: true,
+			}
+			for n := 0; n < tc.ncollisions; n++ {
+				existing[fmt.Sprintf("%s%02d", tc.base, n)] = true
+			}
+			out, err := genUniqueString(tc.base, existing)
+			if tc.errmsg == "" {
+				assert.NoError(err)
+				assert.Equal(tc.exp, out)
+			} else {
+				assert.EqualError(err, tc.errmsg)
+			}
+		})
+	}
+}