detect license ID from full text when incidentally provided as a value (#3876)

---------
Signed-off-by: Christopher Phillips <32073428+spiffcs@users.noreply.github.com>
Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>
Co-authored-by: Alex Goodman <wagoodman@users.noreply.github.com>

internal/licenses/find_evidence_test.go

@@ -0,0 +1,81 @@
+package licenses
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/google/licensecheck"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDefaultScanner_FindEvidence(t *testing.T) {
+	testCases := []struct {
+		name     string
+		fixture  string
+		wantIDs  []string // expected license IDs
+		minMatch int      // minimum # of matches required
+	}{
+		{
+			name:     "Single licenses are able to be recognized and returned Apache 2.0",
+			fixture:  "test-fixtures/apache-license-2.0",
+			wantIDs:  []string{"Apache-2.0"},
+			minMatch: 1,
+		},
+		{
+			name:    "Multiple Licenses are returned as evidence with duplicates at different offset",
+			fixture: "test-fixtures/multi-license",
+			wantIDs: []string{
+				"MIT",
+				"MIT",
+				"NCSA",
+				"Apache-2.0",
+				"Zlib",
+				"Unlicense",
+				"BSD-2-Clause",
+				"BSD-2-Clause",
+				"BSD-3-Clause",
+			},
+			minMatch: 2,
+		},
+	}
+
+	scanner := testScanner()
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			filePath := filepath.Clean(tc.fixture)
+			f, err := os.Open(filePath)
+			require.NoError(t, err)
+			defer f.Close()
+
+			evidence, content, err := scanner.FindEvidence(context.Background(), f)
+			require.NoError(t, err)
+			require.NotEmpty(t, content)
+			require.GreaterOrEqual(t, len(evidence), tc.minMatch, "expected at least %d matches", tc.minMatch)
+
+			var foundIDs []string
+			for _, ev := range evidence {
+				foundIDs = append(foundIDs, ev.ID)
+			}
+
+			require.ElementsMatch(t, tc.wantIDs, foundIDs, "expected license IDs %v, but got %v", tc.wantIDs, foundIDs)
+		})
+	}
+}
+
+func testScanner() Scanner {
+	return &scanner{
+		coverageThreshold: DefaultCoverageThreshold,
+		scanner:           licensecheck.Scan,
+	}
+}
+
+func mustOpen(fixture string) []byte {
+	content, err := os.ReadFile(fixture)
+	if err != nil {
+		panic(err)
+	}
+
+	return content
+}