Fix checksum doesn't match error (#923)

* implement test and replace unused test file

* fix the bug:
The buffer was not containing the full archive because when red by `extract.Archive(...)` some bytes were skipped because of padding.
This was causing the checksum to be different from the expected one

Author: umbynos

main_test.go

@@ -145,7 +145,7 @@ func TestInstallToolV2(t *testing.T) {
 	tests := []test{
 		{bossacInstallURLOK, http.StatusOK, "ok"},
 		{bossacInstallWrongSig, http.StatusInternalServerError, "verification error"},
-		{bossacInstallWrongCheck, http.StatusInternalServerError, "checksum doesn't match"},
+		{bossacInstallWrongCheck, http.StatusInternalServerError, "checksum of downloaded file doesn't match"},
 		{bossacInstallNoURL, http.StatusOK, "ok"},
 	}