fail in __init__() with error if Gtk4 was previously imported

[jwahlstrand]

This prevents Julia from crashing.

Also remove redundant tests that I introduced.

[coveralls]

Pull Request Test Coverage Report for Build 2750738869

• 0 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-2.2%) to 62.77%

---

Files with Coverage Reduction	New Missed Lines	%
src/GLib/signals.jl	6	63.79%

Totals
Change from base Build 2692998865:	-2.2%
Covered Lines:	1659
Relevant Lines:	2643

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2757877506

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+5.3%) to 70.193%

---

Files with Coverage Reduction	New Missed Lines	%
src/GLib/signals.jl	6	74.52%

Totals
Change from base Build 2692998865:	5.3%
Covered Lines:	1785
Relevant Lines:	2543

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #651 (2cbd281) into master (b1e2d5e) will decrease coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #651      +/-   ##
==========================================
- Coverage   65.80%   65.60%   -0.21%     
==========================================
  Files          32       32              
  Lines        2720     2721       +1     
==========================================
- Hits         1790     1785       -5     
- Misses        930      936       +6     

Impacted Files	Coverage Δ
src/Gtk.jl	92.18% <100.00%> (+0.12%)	⬆️
src/GLib/signals.jl	74.18% <0.00%> (-2.46%)	⬇️

---

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