feat: Adding an specific requisite check for pulling the TG image

[rboni-dk]

Added an specific requisite check for pulling the TestGen image.

This is how it looks when the installer can't check the image manifests:

When docker is not running, both requisites fails:

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
dk-installer.py	1232	324	74%	101, 113–125, 158–159, 164–168, 171, 174–189, 192, 195–199, 237–244, 280–281, 323, 373–376, 395–396, 398, 448–477, 481–514, 545–554, 610, 747, 774–779, 851–852, 862, 878–882, 885, 888, 891, 902–935, 1038–1051, 1102–1106, 1117, 1146–1178, 1207, 1249–1250, 1256–1267, 1272, 1345–1347, 1384–1387, 1390–1433, 1442–1450, 1493–1494, 1497–1498, 1503–1506, 1533–1535, 1547–1549, 1575–1579, 1589–1592, 1664–1667, 1693, 1718–1720, 1756, 1775, 1792–1793, 1800, 1802, 1804, 1912–1914, 1931, 1951–1952, 2000, 2064–2103, 2144–2151, 2225–2236, 2243–2251, 2356–2381, 2388–2396, 2405–2459, 2463–2488, 2492–2510
TOTAL	1232	324	74%

Tests	Skipped	Failures	Errors	Time
89	0 💤	0 ❌	0 🔥	0.967s ⏱️