Update Nexus tests to use parallelsuite (#9768)

## What changed?

* Migrate the `NexusTestBaseSuite` to extend a lower-level
`*testcore.TestEnv` instead of `testcore.FunctionalTestBase`, and rename
`NexusTestBaseSuite` to a more aptly-named `NexusTestEnv`
* Migrated all tests that uses the base to `parallelsuite` which
required edits of the following types:
* Change calls like `s.GetTestCluster()` to `env.GetTestCluster()`,
because now these things are provided by the `NexusTestEnv`
**environment** rather than the suite as before
* A lot of tests needed `testcore.WithDedicatedCluster()` for isolation,
otherwise tests using `capture` to assert on metrics would get polluted
* `s.Eventually(...)` (or some other `s.Assertion(...)` type calls) -->
`require.Eventually(t, ...)`, so the failures are reported at the right
test rather than at suite level. Previously tests were not ran in
parallel so this may not have been an issue
* `s.GetTestCluster().FrontendClient()...` --> `env.FrontendClient()...`
    * Creating dedicated cluster env inside subtests instead of outside
    * Misc. changes -- I've commented inline

## Why?

Migrate to the latest and greatest, and make clearer the division
between what's provided by the suite vs. env. This PR also enables
parallel runs of tests, which sped up test runtime (see bottom of PR for
rudimentary benchmarks).

## How did you test it?
- [ ] built
- [ ] run locally and tested manually
- [x] covered by existing tests
- [ ] added new unit test(s)
- [ ] added new functional test(s)

## Test runtimes

**nexus_api_test.go**
**Before:**
```
$ go test ./tests/ -run 'TestNexusApiTestSuite' -count=1
ok      go.temporal.io/server/tests     7.588s
```
**After:**
```
$ go test ./tests/ -run 'TestNexusApiTestSuite' -count=1
ok      go.temporal.io/server/tests     2.799s
```
------------
**nexus_api_validation_test.go**
**Before:**
```
$ go test ./tests/ -run 'TestNexusAPIValidationTestSuite' -count=1
ok      go.temporal.io/server/tests     2.309s
```
**After:**
```
$ go test ./tests/ -run 'TestNexusAPIValidationTestSuite' -count=1
ok      go.temporal.io/server/tests     1.384s
```
-----------
**nexus_workflow_test.go**
**Before:**
```
$ go test ./tests/ -run 'TestNexusWorkflowTestSuite' -count=1
ok      go.temporal.io/server/tests     11.083s
```
**After:**
```
$ go test ./tests/ -run 'TestNexusWorkflowTestSuite' -count=1
ok      go.temporal.io/server/tests     3.709s
```

Author: long-nt-tran