Check for tibble package before using it on tests

[llrs-roche]

Tests on noSuggests flavor of r-hub are failing because tibble is not available:

══ Failed tests ════════════════════════════════════════════════════════════════
── Error ('test-default_split_funs.R:275:3'): trim_levels_to_map split function works ──
Error in `tribble(~ARM, ~RACE, "A: Drug X", "ASIAN", "A: Drug X", "WHITE", 
    "C: Combination", "BLACK OR AFRICAN AMERICAN", "C: Combination", 
    "NATIVE HAWAIIAN OR OTHER PACIFIC ISLANDER")`: could not find function "tribble"
── Error ('test-lyt-tabulation.R:287:3'): ref_group comparisons work ───────────
Error in `tribble(~valname, ~label, ~levelcombo, ~exargs, "A_", "Arm 1", 
    c("A: Drug X"), list(), "B_C", "Arms B & C", c("B: Placebo", 
        "C: Combination"), list())`: could not find function "tribble"
── Error ('test-printing.R:887:3'): showing higher-level ncols works ───────────
Error in `tribble(~valname, ~label, ~levelcombo, ~exargs, "A_C", "Arms A+C", 
    c("A: Drug X", "C: Combination"), list())`: could not find function "tribble"
── Error ('test-subset-access.R:196:3'): make_row_df, make_col_df give paths which all work ──
Error in `tribble(~valname, ~label, ~levelcombo, ~exargs, "A_", "Arm 1", 
    c("A: Drug X"), list(), "B_C", "Arms B & C", c("B: Placebo", 
        "C: Combination"), list())`: could not find function "tribble"

[ FAIL 4 | WARN 0 | SKIP 11 | PASS 923 ]
Error: Test failures

This PR adds skip_if_not_installed where appropriate and also checks that testthat is available before trying to run checks.

[github-actions]

✅ All contributors have signed the CLA
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[llrs-roche]

I have read the CLA Document and I hereby sign the CLA

[shajoezhu]

lgtm! thank @llrs-roche

[github-actions]

Unit Tests Summary

    1 files     28 suites   1m 34s ⏱️
  218 tests   217 ✅ 1 💤 0 ❌
1 579 runs  1 578 ✅ 1 💤 0 ❌

Results for commit f5aa174.

[github-actions]

Unit Tests Summary

    1 files     28 suites   1m 30s ⏱️
  218 tests   217 ✅ 1 💤 0 ❌
1 579 runs  1 578 ✅ 1 💤 0 ❌

Results for commit f5aa174.

[github-actions]

Unit Test Performance Difference

Test Suite	$Status$	Time on main	$±Time$	$±Tests$	$±Skipped$	$±Failures$	$±Errors$
Tabulation framework	💚	$19.46$	$-1.25$	$0$	$0$	$0$	$0$

Results for commit 758b302

♻️ This comment has been updated with latest results.