hard-code gcc for test/sotest binaries

Building the binaries in test/sotest with certain compilers causes the tests to fail with missing shared libraries. For example, with Intel compiler, the Intel shared libraries are not present inside the container:

$ ldd libsotest.so.1
	linux-vdso.so.1 =>  (0x00007fff825fb000)
	libc.so.6 => /usr/lib64/libc.so.6 (0x00002b7396fb1000)
	libimf.so => not found
	libsvml.so => not found
	libirng.so => not found
	libm.so.6 => /usr/lib64/libm.so.6 (0x00002b7397376000)
	libgcc_s.so.1 => /usr/lib64/libgcc_s.so.1 (0x00002b7397678000)
	libintlc.so.5 => not found
	libdl.so.2 => /usr/lib64/libdl.so.2 (0x00002b739788f000)
	/lib64/ld-linux-x86-64.so.2 (0x000055ef5768a000)

Because these binaries will be deployed in a predictable environment, we'll hard-code gcc in test/Makefile instead of using the normally more appropriate $(CC).