It turns out that I didn't wire the LED output to anything, and so yosys optimised the RAM away. If I add

    assign LED = memory_read_data[7:0];

to the test bench, then the RAM is correctly synthesised:

Info: Device utilisation:
Info:            ICESTORM_LC:      27/   5280     0%
Info:           ICESTORM_RAM:       1/     30     3%
Info:                  SB_IO:       9/     96     9%
Info:                  SB_GB:       3/      8    37%
Info:           ICESTORM_PLL:       0/      1     0%