test/system: Use 'command -v' to detect the presence of man(1)

'command -v' is more obvious when reading and is POSIX compatible [1].
While 'hash' also gets the job done, it's more of a caching mechanism.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

https://github.com/containers/toolbox/pull/922

test/system/002-help.bats

@@ -9,7 +9,7 @@ setup() {
 }
 
 @test "help: Run command 'help'" {
-  if ! hash man 2>/dev/null; then
+  if ! command -v man 2>/dev/null; then
     skip "Test works only if man is in PATH"
   fi
 
@@ -20,7 +20,7 @@ setup() {
 }
 
 @test "help: Run command 'help' with no man present" {
-  if hash man 2>/dev/null; then
+  if command -v man 2>/dev/null; then
     skip "Test works only if man is not in PATH"
   fi