Add qemu command prefix to testervecabi tests

[joanaxcruz]

This test suite covers the usage of scalar symbols with auto-vectorisation, to test the GNU ABI
symbols generated by SLEEF.
However, some failures are recorded when emulating these tests using user mode QEMU, for a
cross-compile build for AArch64 and s390x.
This is because this test suite does not get called with the EMULATOR command necessary.
This fix adds the emulator to the test command
which makes the tests pass when cross-compiling
and using user mode QEMU emulation.

[joanaxcruz]

A bit more context on this PR:

Looking at the logs from running
ctest --output-on-failure --test-dir build-aarch64-gcc
on a aarch64 cross build on a x86 host where we pass -DEMULATOR=qemu-aarch64:

Before these changes, testervecabit suite would be invoked like:
Test command: "$(PWD)/sleef/build-aarch64-gcc/bin/testercevabi"

For some reason when these tests were being run in a docker environment, the following error would show:

After these changes, the invocation of testervecabi becomes:
Test command: /usr/bin/qemu-aarch64 "$(PWD)/sleef/build-aarch64-gcc/bin/testercevabi"

We stop observing the error above.

[blapie]

Maybe worth mentioning that these failures were not observed on GHA since QEMU is invoked on a system level, and no EMULATOR is passed to cmake.
Maybe something to change in the future, so emulation is enabled as expected by build system, i.e. via a prescribed EMULATOR.