[FR]: `EXPECT_NONFATAL_FAILURE` and friends: Support multiple (variadic) substrings

I would like to be able to pass multiple substrings to `EXPECT_NONFATAL_FAILURE` so that all the substrings must appear in the failure output.

This should I guess also apply to the other `EXPECT_(FATAL|NONFATAL)_FAILURE[_ON_ALL_THREADS]`.

### Does the feature exist in the most recent commit?

No

### Why do we need this feature?

When testing matchers that produce detailed output (e.g. multi-line, or printing details from inner/nested matchers), it's brittle and cumbersome to test either one big string (where the test now depends on unimportant indentation/formatting), or have to repeat the entire `EXPECT_NONFATAL_FAILURE` multiple times to verify multiple substrings.

### Describe the proposal.

For my use case, I want to be able to do something like:

```
EXPECT_NONFATAL_FAILURE(
[&] {
EXPECT_THAT(myObject, MyMatcher(...));
}(),
"Substring 1",
"Substring 2");
```
I haven't looked at whether this can be done in a backward compatible way using `__VA_ARGS__`/`__VA_OPT__`/etc.

### Is the feature specific to an operating system, compiler, or build system version?

No.

Start by locating the implementations of EXPECT_NONFATAL_FAILURE and the related EXPECT_(FATAL|NONFATAL)_FAILURE[_ON_ALL_THREADS] macros, then trace how their failure substrings are checked. Done means each macro accepts multiple substrings, requires every substring to appear, and preserves existing one-substring usage across the supported variants.

Written by the indexing model from the issue text.