all: fix go vet warnings

common/compiler/solidity_test.go

@@ -57,7 +57,7 @@ func TestCompiler(t *testing.T) {
 	}
 
 	if len(contracts) != 1 {
-		t.Errorf("one contract expected, got\n%s", len(contracts))
+		t.Errorf("one contract expected, got %d", len(contracts))
 	}
 
 	if contracts["test"].Code != code {

common/math/dist_test.go

@@ -35,7 +35,7 @@ func TestSum(t *testing.T) {
 	summer := summer{numbers: []*big.Int{big.NewInt(1), big.NewInt(2), big.NewInt(3)}}
 	sum := Sum(summer)
 	if sum.Cmp(big.NewInt(6)) != 0 {
-		t.Errorf("not 6", sum)
+		t.Errorf("got sum = %d, want 6", sum)
 	}
 }
 

common/path.go

@@ -34,7 +34,7 @@ func MakeName(name, version string) string {
 
 func ExpandHomePath(p string) (path string) {
 	path = p
-	sep := fmt.Sprintf("%s", os.PathSeparator)
+	sep := string(os.PathSeparator)
 
 	// Check in case of paths like "/something/~/something/"
 	if len(p) > 1 && p[:1+len(sep)] == "~"+sep {