bug(Tooltip): Harness returns if _any_ tooltip is open

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

`MatTooltipHarness.isOpen()` returns `true` if _any_ tooltip is open. Intuitively it should returns if the tooltip _associated with that harness_ is open. The docs also sound like that's what it should do, because it uses the word "the" instead of "a":

> Gets whether the tooltip is open.

### Reproduction

Steps to reproduce:
1. Have tooltips on 2 elements
2. Open one of them
3. Test `.isOpen()` on the harness for a different one

Here's a stackblitz: https://stackblitz.com/edit/harness-issue-84mcjr?file=src/app/button-harness-example.spec.ts

### Expected Behavior

`.isOpen()` would return true for the tooltip that is open, and false for the one that is not

### Actual Behavior

`.isOpen()` returns true for both

### Environment

```
@angular-devkit/architect 0.1501.2
@angular-devkit/build-angular 15.1.2
@angular-devkit/core 15.0.5
@angular-devkit/schematics 15.0.5
@angular/cdk 15.0.3
@angular/cli 15.0.5
@angular/fire 7.5.0
@angular/material 15.0.3
@schematics/angular 15.0.5
rxjs 7.5.7
typescript 4.8.4
```

Start by locating MatTooltipHarness.isOpen() and the associated tooltip harness tests. Reproduce the two-tooltip case from the issue, then verify that isOpen() distinguishes the tooltip associated with the harness from other open tooltips; the relevant test should pass for both the open and closed harnesses.

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