fix: test case for interest_is_interesting_test.cpp

[mhdadk]

The following test case contains an incorrect want value.

cpp/exercises/concept/interest-is-interesting/interest_is_interesting_test.cpp

Lines 169 to 174 in acc3255

	TEST_CASE("Years before large difference between start and target balance", "[task_4]") {
	double balance{2345.67};
	double target_balance{12345.6789};
	int want{85};
	REQUIRE(years_until_desired_balance(balance, target_balance) == want);
	}

Currently, the test case has the following parameters:

• $\texttt{balance} = 2345.67$
• $\texttt{target\_balance} = 12345.6789$
• $\texttt{want} = 85$

However, the value of want is incorrect. Since $\texttt{balance} = 2345.67$, then, as given in the instructions, the interest rate is $\% 1.621$. If we assume that $\texttt{want} = 85$ as given above, then the final balance after $\texttt{want}$ years is

$$\begin{align} \texttt{balance} \times \left(1 + \frac{\verb|interest_rate|}{100}\right)^\texttt{want} &= 2345.67 \times \left(1 + \frac{1.621}{100}\right)^{85} \\\ &\approx \$ 9201.558 \end{align}$$

Similarly, the final balance after $\texttt{want} - 1$ years is approximately $\$ 9054.78$. If the want value was correct, then the target balance would have been between the final balance after want - 1 years and the final balance after want years. However, this is not the case.

This PR fixes the want value by setting it to $104$ years. We can see that this is indeed correct since

$$\begin{align} 2345.67 \times \left(1 + \frac{1.621}{100}\right)^{103} &\approx \$ 12,290.334 \\\ &< \verb|target_balance| = 12345.6789 \\\ &< 2345.67 \times \left(1 + \frac{1.621}{100}\right)^{104} \approx \$ 12,489.561 \end{align}$$

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[mhdadk]

FYI: I posted the relevant forum discussion here.