1) For $1< p< \infty$. Let $M = \sup \|f_n\|_{L^p}$. Since $f_n \to f$ in measure, hence there is a subsequence $f_{n_k}$ which converges a.e. to $f$. By Fatou's lemma, we have $f \in L^p$ and $\|f\|_{L^p} \leq M$. We need to check
$$\lim_{n\to \infty} \int_0^1 f_n g dx = \int_0^1 f g dx,\qquad\qquad (*)$$
for any $g \in L^{p'}$ with $p' = p/(p-1)$. Indeed, for any $\epsilon >0$, we have $\lim_{n\to \infty} m(\{|f_n -f| > \epsilon\}) = 0$ and
$$|\int_0^1 (f_n -f)g dx| = |\int_{\{|f_n -f| >\epsilon\}} (f_n -f) g dx + \int_{\{|f_n -f| \leq \epsilon\}} (f_n -f) g dx,$$
and hence
$$|\int_0^1 (f_n -f)g dx| \leq \int_{\{|f_n -f| >\epsilon\}} |f_n -f| |g| dx + \epsilon \int_{\{|f_n -f| \leq \epsilon\}} |g| dx.$$
By Holder inequality, we have
$$\int_{\{|f_n -f| \leq \epsilon\}} |g| dx \leq \int_0^1 |g| dx \leq \|g\|_{L^{p'}},$$
and
$$\int_{\{|f_n -f| >\epsilon\}} |f_n -f| |g| dx \leq (\int_{\{|f_n -f| >\epsilon\}} |f_n -f|^p dx)^{\frac1p}(\int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx)^{\frac1{p'}} \leq 2M (\int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx)^{\frac1{p'}}.$$
Therefore, it holds
$$|\int_0^1 (f_n -f)g dx| \leq 2M (\int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx)^{\frac1{p'}} + \epsilon \|g\|_{L^{p'}}.\qquad\qquad (1)$$
We claim that
$$\lim_{n\to \infty} \int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx = 0. \qquad\qquad (**)$$
Indeed, for any $\delta >0$, there exists $A >0$ such that $\int_{\{|g| > A\}} |g|^{p'} dx < \delta$. Hence
$$\int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx \leq \int_{\{|f_n -f| >\epsilon\}\cap\{|g| > A\}} |g|^{p'} dx + \int_{\{|f_n -f| >\epsilon\}\cap\{|g| \leq A\}} |g|^{p'} dx \leq \delta + A^{p'} m(\{|f_n -f| > \epsilon\}).$$
Letting $n \to \infty$ and using $m(\{|f_n -f| > \epsilon\}) \to 0$, we get
$$\limsup_{n\to \infty} \int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx \leq \delta,$$
for any $\delta >0$. Consequently, we get
$$\limsup_{n\to \infty} \int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx \leq 0.$$
Evidently, we have $\liminf_{n\to \infty} \int_{\{|f_n -f| >\epsilon\}} |g|^{p'} dx \geq 0$. Hence, the claim $(**)$ holds.
Letting $n \to \infty$ and using the claim $(**)$, we obtain
$$\limsup_{n\to \infty} |\int_0^1 (f_n -f)g dx| \leq \epsilon \|g\|_{L^{p'}},$$
for any $\epsilon >0$. Therefore, it holds
$$\limsup_{n\to \infty} |\int_0^1 (f_n -f)g dx| \leq 0.$$
Evidently, we have $\liminf_{n\to \infty} |\int_0^1 (f_n -f)g dx| \geq 0$. Hence,
$$\lim_{n\to \infty} \int_0^1 (f_n -f) gdx =0,$$
which proves $(*)$.
2) For $p =1$, the conclusion does not holds. For example, consider $f_n = n \chi_{(0,1/n)}$, we have $\|f_n\|_{L^1} =1$ for any $n$ and $f_n \to 0$ in measure. However, we have $1 \in L^\infty = (L^1)^*$ and
$$\int_0^1 f_n \, 1 dx = 1 \not\to 0.$$