test(unit/strings_spec): provide context for vim_snprintf tests

Since these assertions all use a common function to perform the test
assertions, it's difficult to figure out which test failed:

    ERROR    test/unit/testutil.lua @ 785: vim_snprintf() positional arguments
    test/unit/testutil.lua:757: test/unit/testutil.lua:741: (string) '
    test/unit/strings_spec.lua:143: Expected objects to be the same.
    Passed in:
    (number) 6400
    Expected:
    (number) 6'
    exit code: 256

Adding context to the assertion makes it clearer what the problem is:

    ERROR    test/unit/testutil.lua @ 785: vim_snprintf() positional arguments
    test/unit/testutil.lua:757: test/unit/testutil.lua:741: (string) '
    test/unit/strings_spec.lua:149: snprintf(buf, 0, "%1$0.*2$b", cdata<unsigned int>: 0xf78d0f38, cdata<int>: 0xf78dc4e0) = 001100
    Expected objects to be the same.
    Passed in:
    (number) 6400
    Expected:
    (number) 6'
    exit code: 256

test/unit/strings_spec.lua

@@ -140,7 +140,13 @@ end)
 
 describe('vim_snprintf()', function()
   local function a(expected, buf, bsize, fmt, ...)
-    eq(#expected, strings.vim_snprintf(buf, bsize, fmt, ...))
+    local args = { ... }
+    local ctx = string.format('snprintf(buf, %d, "%s"', bsize, fmt)
+    for _, x in ipairs(args) do
+      ctx = ctx .. ', ' .. tostring(x)
+    end
+    ctx = ctx .. string.format(') = %s', expected)
+    eq(#expected, strings.vim_snprintf(buf, bsize, fmt, ...), ctx)
     if bsize > 0 then
       local actual = ffi.string(buf, math.min(#expected + 1, bsize))
       eq(expected:sub(1, bsize - 1) .. '\0', actual)