testutils: fix determinism in random predecessor history tests

Previously, the `rng` instance used by the tests was initialized once
in the `var` block. That meant that every assertion that relied on
that rng was order-dependent: running a single test in that file in
isolation would lead to failed assertions.

This commit ensures that every test resets the rng before making any
assertions, guaranteeing we get the same values regardless of
execution order.

Epic: none

Release note: None

pkg/testutils/release/releases_test.go

@@ -40,10 +40,17 @@ var (
 		},
 	}
 
-	// Results rely on this constant seed
-	rng = rand.New(rand.NewSource(1))
+	// Results rely on this constant seed.
+	seed = int64(1)
+
+	// Tests must call `resetRNG` if they use this global rng.
+	rng *rand.Rand
 )
 
+func resetRNG() {
+	rng = rand.New(rand.NewSource(seed))
+}
+
 func TestLatestAndRandomPredecessor(t *testing.T) {
 	testCases := []struct {
 		name           string
@@ -72,16 +79,17 @@ func TestLatestAndRandomPredecessor(t *testing.T) {
 			name:           "latest is withdrawn",
 			v:              "v22.2.3",
 			expectedLatest: "22.1.12",
-			expectedRandom: "22.1.8",
+			expectedRandom: "22.1.10",
 		},
 	}
 
-	for _, tc := range testCases {
-		oldReleaseData := releaseData
-		releaseData = testReleaseData
-		defer func() { releaseData = oldReleaseData }()
+	oldReleaseData := releaseData
+	releaseData = testReleaseData
+	defer func() { releaseData = oldReleaseData }()
 
+	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			resetRNG() // deterministic results
 			latestPred, latestErr := LatestPredecessor(version.MustParse(tc.v))
 			randomPred, randomErr := RandomPredecessor(rng, version.MustParse(tc.v))
 			if tc.expectedErr == "" {
@@ -119,23 +127,24 @@ func TestLatestPredecessorHistory(t *testing.T) {
 			v:              "v23.1.1",
 			k:              3,
 			expectedLatest: []string{"19.2.0", "22.1.12", "22.2.8"},
-			expectedRandom: []string{"19.2.0", "22.1.12", "22.2.1"},
+			expectedRandom: []string{"19.2.0", "22.1.8", "22.2.8"},
 		},
 		{
 			name:           "with pre-release",
 			v:              "v23.1.1-beta.1",
 			k:              2,
 			expectedLatest: []string{"22.1.12", "22.2.8"},
-			expectedRandom: []string{"22.1.0", "22.2.5"},
+			expectedRandom: []string{"22.1.8", "22.2.8"},
 		},
 	}
 
-	for _, tc := range testCases {
-		oldReleaseData := releaseData
-		releaseData = testReleaseData
-		defer func() { releaseData = oldReleaseData }()
+	oldReleaseData := releaseData
+	releaseData = testReleaseData
+	defer func() { releaseData = oldReleaseData }()
 
+	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			resetRNG() // deterministic results
 			latestHistory, latestErr := LatestPredecessorHistory(version.MustParse(tc.v), tc.k)
 			randomHistory, randomErr := RandomPredecessorHistory(rng, version.MustParse(tc.v), tc.k)
 			if tc.expectedErr == "" {