Core: Add automatic labels in test.each() for primitive values in arrays

[Krinkle]

Fixes #1733.

Before

TAP version 13
ok 1 array of strings [0]
ok 2 array of strings [1]
ok 3 array of strings [2]
ok 4 array of strings [3]

ok 13 array of mixed [0]
ok 14 array of mixed [1]
ok 15 array of mixed [2]
ok 16 array of mixed [3]
ok 17 array of mixed [4]
ok 18 array of mixed [5]
ok 19 array of mixed [6]
ok 20 array of mixed [7]
ok 21 array of mixed [8]
ok 22 array of mixed [9]
ok 23 array of mixed [10]

ok 27 keyed objects [caseFoo]
ok 28 keyed objects [caseBar]

After

TAP version 13
ok 1 array of strings [foo]
ok 2 array of strings [xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
ok 3 array of strings [$]
ok 4 array of strings [http://example.org]

ok 13 array of mixed [0: undefined]
ok 14 array of mixed [1: null]
ok 15 array of mixed [2: false]
ok 16 array of mixed [3: true]
ok 17 array of mixed [4: 0]
ok 18 array of mixed [5: 1]
ok 19 array of mixed [6: -10]
ok 20 array of mixed [7: 3.3333333333333335]
ok 21 array of mixed [8: 1e+43]
ok 22 array of mixed [9: Infinity]
ok 23 array of mixed [10: NaN]

ok 27 keyed objects [caseFoo]
ok 28 keyed objects [caseBar]

\cc @vtintillier

[vtintillier]

Nice, thanks a lot!