test: add additional tests for a strptime()

The testsuite "Datetime string parsing by format (detailed)"
tests parsing of a string with various conversion specifications.
However, `tostring(dt)` is used as a test oracle, and all these
testcases do not test some conversion specifications at all
because the metamethod `__tostring` for the datetime object uses
`:format()` with the default format string.

Due to missed tests for conversion specifications, the behavior
described in #10470 was missed:

```
tarantool> dt = date.parse('Mon', {format = '%a'})
tarantool> dt
---
- 1970-01-01T00:00:00Z
...

tarantool> dt:format('%a')
---
- Thu
...
```

The patch adds tests for `strptime` with all possible conversion
specifications described in strftime(3) [1][2].

1. https://man.freebsd.org/cgi/man.cgi?strftime(3)
2. https://pubs.opengroup.org/onlinepubs/009695399/functions/strptime.html

Follows up #8588
Follows up #6731
Relates to #10470

NO_CHANGELOG=testing
NO_DOC=testing

(cherry picked from commit 109b18df4da0b34f4408f67902e0db04540cb32b)