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GH-44483: [GLib] Don't check invalid decimal precision message in test (#44484)

### Rationale for this change

Two tests failed if `-DARROW_EXTRA_ERROR_CONTEXT=ON` in the C++ build.

### What changes are included in this PR?

Don't check invalid decimal precision message in test. Because it depends on `-DARROW_EXTRA_ERROR_CONTEXT={ON,OFF}`. Our tests should be stable whether the option value. 

Before this change: compare the exception class and message.
After this change: compare the exception class only.

### Are these changes tested?

YES

Before this change

```
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1962 tests, 2148 assertions, 2 failures, 0 errors, 0 pendings, 14 omissions, 0 notifications
99.8973% passed
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
23.71 tests/s, 25.95 assertions/s
```

After this change

```
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1962 tests, 2148 assertions, 0 failures, 0 errors, 0 pendings, 14 omissions, 0 notifications
100% passed
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
23.75 tests/s, 26.00 assertions/s
```

### Are there any user-facing changes?

No.
* GitHub Issue: #44483

Co-Authored-by: Sutou Kouhei <kou@ clear-code.com>

Authored-by: Hiroyuki Sato <hiroysato@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>