Upkeep: Standardize test file names to avoid "mytest" and "migrated shinytest" names

[gadenbuie]

There's enough tooling around tests that this isn't a blocker, but I find the "mytest" context a bit weird in our tests. I'd much rather we have test-026-shiny-inline.R than test-mytest.R in inst/apps/026-shiny-inline/tests/testthat/.

Running tests for app: 026-shiny-inline

Loading required package: shiny
Loading required package: testthat
✔ | F W S  OK | Context

⠏ |         0 | mytest                                                          
⠋ | 1       0 | mytest                                                          
⠙ | 1       1 | mytest                                                          
⠹ | 2       1 | mytest                                                          
⠸ | 2       2 | mytest                                                          
⠼ | 3       2 | mytest                                                          
⠴ | 3       3 | mytest                                                          
✖ | 3       3 | mytest [7.1s]
────────────────────────────────────────────────────────────────────────────────
Failure ('test-mytest.R:13:3'): Migrated shinytest test: mytest.R
Snapshot of `file` to 'mytest/001.json' has changed
Run `testthat::snapshot_review('mytest/')` to review changes
Backtrace:
    ▆
 1. └─app$expect_values() at test-mytest.R:13:2
 2.   └─shinytest2:::app_expect_values(...)
 3.     └─shinytest2:::app__expect_snapshot_file(...)
 4.       ├─base::withCallingHandlers(...)
 5.       └─testthat::expect_snapshot_file(...)

One small advantage is that it'd be (marginally) easier to open the test files, e.g. test-026... to jump into the test file.